I vote don't worry about backward compatibility and make the change.

Is this a future plan for after the alpha release, or ate you looking at implementing this soon?  Frankly, I think working at backwards compatibility to make trees look perfect isn't worth your time.  For individual games, I think people will dismiss it.  On servers, well, I imagine most of the server world will have been viewed and determined anyways.  Eventually, the issue will be moot.

if you can make an AI like that, why dont other games have such a thorough system?

They kind of do.  GOAP (Goal Oriented Action Planning) is becoming more common.
http://web.media.mit.edu/~jorkin/goap.html

I don't know how strategic I'll be able to make it yet, but I've found it's mostly how the goals are encoded.  I kind of have a loose multi-layer design at this point.  It may be too slow and I'll fall back on more conventional high-level thinking with goap for the tactical level.  But we'll see.

Either way, if an NPC wanted to get into a house (the want already being decided in some way) and one of the actions he has available is "bash down the door" then that will be considered in his tactical planning.

Based on my understanding of GOAP, either actions need to be mapped as relevant to a certain goal by the designer, or the NPC needs to discover the relvancy through testing a success.  For a FPS/TPS like F.E.A.R. this seems manageable because ultimately the actions of every enemy distills down to, kill the player.  So the designers were able to specifically map certain actions to tactical (shoot, move to nearest cover) or objective (attack, vacate compromised position) goals.

I imagine that the list of actions available to an NPC are enormous.  While preconditions can filter the list down, it's still an incredible feat to perform AI planning through a tangled web of destroying the door, finding a key, or even buying the services of a locksmith.  And then all the tactics to accomplish such objectives.

How are you programming the NPCs to negotiate through their available actions?  Are you limiting them to only certain sets of actions per goal?  Or is there a web of action and goal relationships with heuristic values to determine their cost and effectiveness?

Are you planning on modeling ship stability? Could ships capsize?  ( I have a small background in naval architecture and nautical science from college...so I'm geeking out. ) Is it possible to have naval battles?

I'd imagine if the answer to all is yes, you'd have in game recreations of the Vasa. http://en.wikipedia.org/wiki/Vasa_(ship)

Which would be hilarious.  It could lead to a whole in game profession of a ship designer. If your character has the skill then you would get in game feedback on the stability.  If your character doesn't, well...good luck.

Awesome.

I know I'm entering the discussion late.  Sorry, I just discovered the game this week.

I always liked skills that had a reflection of player skill.  For example, if your careful enough as a player in the lock picking interface in Skyrim, you as a "novice" could pick advanced locks.

I think it would positively impact the game economy if complex crafting, like sword making required skillful player input.  The more precise the player is, the better the crafting result is.  As a demonstration of the concept, Epic Mickey for the 3ds has the player trace drawings to help Mickey draw new items.  The more accurately the player traces, the better the resulting item.

I could see this similar to an interface for skinning a deer.

I know this would add a lot to the programming and possibly server overhead, but I think it would add a lot of value to player crafted items.