How This Topic Came About
As most people know, the way Frog God games is able to offer inexpensive old school modules is because of the effective subsidy that comes from Pathfinder players. It also works the other way around -- both game systems "share" the cost of a module's production -- but from our perspective it still operates as a subsidy.
This happens because a FGG module is initially produced for one system, then is also adapted for the other. The end results can be quite different, but they can share the same interior art, which cuts that part of the cost per module in half.
How This Creates a Judgment Call
The adaptations can run very deep in terms of the changes being made -- they aren't just a set of mechanical changes to stats. As a result, there are some judgment calls to be made, and what's particularly interesting is that these judgment calls require the adapter of the module to have a fairly clear idea of what the parameters of "old school" really are in terms of creating a real document for a real DM to use at a real gaming table. In other words, what do you need to change and why?
Many Pathfinder modules (and this is true for 3e as well) rely on the concept of powering up the "standard form" of a monster in one of a few various ways: size (obvious effect); templates (adding a set of skills and attributes based on a concept like "cursed," or "shadow"); or character levels. This is something that appears in old-school D&D as well, either in a fairly random sense ("THIS skeleton breathes fire, because ... it's ... well, it's a FIRE BREATHING skeleton!" -- no real explanation needed) or in a relatively formalized sense ("Drow have cleric levels") or based on a simple "some are stronger" concept (most humanoid monsters have leader types with more hit dice).
So it can't be said that OD&D or AD&D are totally different from 3e/PF at a fundamental level when it comes to monster advancement. The 3e/PF advancements are all subject to official monster-advancing rules, which has a "wrong" feeling for me, but from the players' perspective, that's a hidden factor. They see only the monster's outer facade, not the webwork of rules and numbers of the actual stats. What they perceive from their perspective is roughly similar to what we, as OD&D/AD&D players, see from our side of the DM screen as well. That is to say, monsters with abilities over and above the standard "type" for that monster.
However, there is a difference, I think, in terms of when monster advancement is used. This gets into that unpleasant territory where everything I say is subject to a, "Yeah, well, that's how you do it, buster" type of criticism; where I appear to be making vast generalizations; and where I might sound like I'm criticizing a "new school" attitude by means of a straw man argument. My point, though, is that when you are making a conversion from a modern version of D&D into an older version, if you want to do it right, without using rote formulas, this is exactly the territory you must enter.
I'm going to make the sweeping generalization that 3e and PF use monster advancement in two particular ways that are different from the old style. First, monster advancement is treated as more common -- that is to say, monsters are generally seen as having more variation in size, HD, and skills across the entire species, whereas old style monsters are seen as pretty similar within one species. Result? The standard type tends to disappear in 3e/PF into little more than one of the possibilities -- it exists only in the monster manual as a baseline for modification. This happens for various reasons related to the structure of the games: 3e/PF generally don't promote mass combats because of the complexity of combat (at a structural level, PF is closer to a skirmish wargame, while older style leans a bit more toward squad-level rules). Also, the combat-balancing rules internal to PF/3e mandate giving the DM more tools for fine-tuning the difficulty of an encounter. There are a bunch of other structural "nudges" toward monster advancement and the way it is used, but since I'm already generalizing with flamboyant abandon, I won't make an artificial excursion into the reasons.
The conclusion to be drawn from the previous paragraph is that when converting a 3e/PF module, I have to be alive to the fact that in order to maintain an old-style feel to the module, one of my tools is to back down the number of individualized versions of any particular monster type. Increase the number of "regular" ones. Sometimes by quite a bit, which leads me to a second point:
In addition to the commonness of advanced monsters in PF, the degree to which they are advanced is a huge difference. In fact, I think it's by far the more significant distinction between the systems. You can, without blinking, have an orc that's a 10th level fighter in PF. For old-style gaming, that simply strains not only the traditional method, but the nature of the monster. Orcs are fodder, attacking in masses with their "reckless hate." That's the feel of the monster. They aren't individually very powerful, not even the chieftains. There's an upper limit to the concept of "an orc." If you need something more powerful to throw into the mix, you don't advance the orcs by much -- what you do is to add in some ogres, perhaps an ogre mage, perhaps a (still-weak) witch doctor, perhaps even a renegade human character-type. There are plenty of ways to increase the difficulty of an orc encounter, but generally it's done without advancing the orcs -- and CERTAINLY not by upping the power of ALL the orcs in the encounter. We don't tend, except in those situations where the rule is made to be broken, the entire power level of a monster type.
Hence, a second tool for converting a PF module (when faced with an encounter in which all the monsters are stepped up) is to create two or more tiers of power. This means adding grunts, leader types, and possibly a spell caster so that the tactics of the players have to account for the varying capabilities of these different types. The tactical side of an encounter, to my mind, is vastly enriched by having a mix of opponent capabilities. This is still the case in many PF encounters, I should point out, but what I'm talking about is making those tiers VERY distinct from each other in terms of what they can do. The differences between different opponents in a PF encounter is relatively more subtle than it is in an old-style encounter. In the old-style approach, you often see a very big distinction between (a) the rank-and-file who charge the party, weak but hoping to overcome by numbers -- or else showering arrows down and dying in droves at the same time; (b) the enemy spellcasters who rely on the others to shield them from melee, and (c) the tougher hardpoints, like ogres in an orc encounter, or the orc sergeants.
The big distinction between combat tiers in terms of tactics and capabilities is a distinct characteristic of old-style encounters -- again, partly because the system is more capable of quickly handling combats that involve masses of grunts.
Okay, that's all for now, and I really only covered one issue -- the advancement of monsters. Probably I'll weigh in later with something about encounter/game balance, the "feel" of levels, traps/locks, and other matters that are involved with conversion.
Just to leave you with this point, though -- I'm not so much talking about the methods of conversion as I am talking about what the conversions tell me about what I think old-style gaming really is. What defines it -- what makes me actually alter something in a PF module to make it conform to an old-school style, even though I'm totally aware that someone else might approach it completely differently.
And so, until next time! Same bat channel, and all that stuff.