I believe the formal definition here is that older products, like the original 3.x psionics or Book of Nine swords books, would work fine within the current framework. That sounds simple and easy in concept, but it makes one wonder why some things changed so drastically while other stayed the same. For example, the base classes all kept the same number of skill points per level, but many of the lists of class skills were modified. Also, BAB and save progression stayed the same across the board, but rogues, bards, and full arcane casters had their hit points per level changed. The way caster level is handled for purposes of concentration was changed pretty dramatically. Feats and ability score bonuses are more frequent. All that, and we haven't even touched the numerous spell, magic item, feat, and miscellaneous rule changes made.
So, that's a lot of changes made to the system, both in it's finer, situational points and in how some of the larger, universal matters work. Does this mean it's still really 'backwards compatible' with 3.5? When you require a spreadsheet to modify all the monster/NPC statistics and a small guidebook of fine-point rules changes to make sure things work appropriately, is that really what 'compatible' means? If so, what is necessary to break that degree of compatibility? Would it require complete rewrites of each class that add in a new ability per level? An entirely new magic / supernatural ability system? Or is it not anything specific that can be named and more an almost ephemeral 'feel' of how the game plays? Curious about people's attitude towards this, ESPECIALLY any developers who use this principle as a guide for developing new Pathfinder content.
Sadly, all the classes had to be modified somehow, else Paizo may have breached the open game license. On the plus side, the changes made some sense for the changes.
Why the Hit Point changes? My guess is that Paizo wanted to equate Hit die types with the BAB progression type (The Barbarian is a notable exception). Now everyone with a 1/2 BAB has a D6, a 3/4 BAB has a D8, and a full BAB has a D10 (except the Barbarian's D12).
There is a free Conversion booklet that allows people to convert 3.5 characters, beasts, etc. to Pathfinder. Many of the game mechanics are very similar. In light of D&D recent reprints, this becomes important. It should also be noted that some of Paizo's earlier Pathfinder products were made before the Core Rulebook and were meant to be used with D&D 3.5 (such as the Original Rise of the Runelords, Second Darkness, Curse of the Crimson Throne, and Legacy of Fire Adventure Paths).
Why the Core Rulebook? Because Paizo wanted to continue making Pathfinder products at a time when WOTC / D&D changed the open license agreement for D&D 4.0 (Notice the drop of third party support for 4.0? That was because of the new license agreement, not because of lack of interest or adaptation to the new rules).
In short, not perfect, yet certainly more compatible than say - Palladium Fantasy, or GURPs, or White Wolf / World of darkness, D&D versions 1-2 or 4, etc.
Pathfinder is still backwards compatible with 3rd edition, despite all that was changed. Backwards compatible, in reference between those 2, basically means a fairly small amount of conversion between them.
Pathfinder is as backwards compatible with 3.5 as 3.5 is with 3.0. However, don't expect much in the way of using the 3rd edition D&D stuff with your Pathfinder games (unless you are the DM, of course), as technically it's a 3PP, and almost all DMs disallow it.
Sadly, all the classes had to be modified somehow, else Paizo may have breached the open game license. On the plus side, the changes made some sense for the changes.
Why didn't the NPC classes have to be changed, then? Adepts and warriors function precisely as they did in 3rd edition. They're part of the CRB, too. If changes were not made to something, is it because of the desire for backwards compatibility on the part of Paizo? That's the main point here. The NPC classes certainly feature into elements of Paizo's first four adventure paths, just like some of the monster types that weren't changed from 3.5.
Why the Hit Point changes? My guess is that Paizo wanted to equate Hit die types with the BAB progression type (The Barbarian is a notable exception). Now everyone with a 1/2 BAB has a D6, a 3/4 BAB has a D8, and a full BAB has a D10 (except the Barbarian's D12).
Agreed, that part seems quite obvious (except for the barbarian, of course). The question is, why change that over, say skill points allotments, or saves? Which, if any, of these choices had to do with maintaining backwards compatibility?
Why the Core Rulebook? Because Paizo wanted to continue making Pathfinder products at a time when WOTC / D&D changed the open license agreement for D&D 4.0 (Notice the drop of third party support for 4.0? That was because of the new license agreement, not because of lack of interest or adaptation to the new rules).
I'm not asking about WHY they wanted backwards compatibility, that much is clear. It's the specifics of what that compatibility means that interest me. Which parts are essential to maintaining that status or, if it's more the totality of changes reaching a breaking point, how much does each game element factor in?
"Backwards compatible" means that you, as a group, could use prior materials in a Pathfinder game without a lot of effort if you so chose. I've found this to be mostly true for low-level stuff. For higher level adventurers, I'll typically end up restatting every NPC and every monster that doesn't appear in one of the Pathfinder bestiaries, and that takes much longer.
For classes, prestige classes, and the like, it takes precious little time to make it Pathfinder-usable. Whether it's Pathfinder-balanced is a whole different question. Since a lot of Pathfinder options are sometimes viewed as "not Pathfinder-balanced", balancing third party stuff to the group's satisfaction can be challenging.
But I have no qualms about picking up a 3.5 module and doing the necessary conversions if the module appeals to me. I haven't seen much 3.0 material (sorry, Tweet, I didn't get back into D&D until after the 3.0 years), so I have no idea how well that works.
As far as class changes: The classes were changed to encourage single-classing, something strongly discouraged by the 3.5 mechanics. This required adding a bit of a carrot to the classes most likely to be multiclassed in 3.5. This worked to some extent (more for some classes, less for others). The changes were also made to avoid the phenomenon of "dead levels" where a level-up might impart hit points, skill points, and other "boring" improvements but nothing else. Now you'll at least get something useful every level in most cases, whether it's a feat, an improvement to a class ability, or a new spell level available to you. I don't think license had much to do with it, but IANAL (and IANAD).
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Well, lets see. If you play Pathfinder, and a DM wants to run a 3.5 adventure, how well does it work out?
Pretty easy. Even if monsters received a few changes, you can run them just as they were published just fine.
Can you port over feats/spells/PrC with minimal work?
Absolutely!
Yeah, I think they are still close enough to be considered backwards compatible.