tbaMUD Continued development of the CircleMUD core

I am working on building rooms and objects in the MUD (TBA code). If I make changes to the actual code (say, changing classes or races) how do I make sure not to mess with what I have built in the game (rooms, mobs, objs etc)?

Is there a separate file for rooms and suchlike built inside the game itself?

All the World files are in the lib/world directory. Changing races/classes will not affect these files so you have nothing to worry about. Only when you change room/mob/obj/trig/etc. formats will you need to update how the World files are saved or read.

So as long as I do not mess with lib/world, I won't hurt any of the changes I have made in-mud?

Correct. As a safety measure though, you should do backups regularly. Another good idea is using something like SVN or GIT to track changes. If you do break something, it's easier to revert back to an older version that worked. There's a bit of a learning curve getting used to version trackers, if you don't want to go that route, a simple backup is sufficent :).