Short answer: Generate a new world. If you really really want to keep your current world, there are a few rules:
Virtually all entity
changes whatsoever require a new world. Some don't, and some only do when certain conditions are met. It would be impossible to document every case here.
Adding/removing entire objects requires a new world. NOTICE: Changing an object's identifier does both of these.
to tags except those in entity objects do not require a new world. NOTICE: Some, such as biomes, are highly relevant during worldgen and may not entirely have the desired effect.
Make sure you make your changes in the copy of the raws included in the region's /data/save folder instead of the /raw/objects folder, which is only used during new-world-generation.
If you plan on making a new world, obviously, change the base raws.Play it safe and generate a new world if you're unsure.
Glossary of terms:
entity -- Refers to an object from any file with a header of [OBJECT:ENTITY]; the same lexicon is true for all other variations of OBJECT:X
tag -- A singular bracketed item that defines a characteristic of an object. Example: [SPEED:900]
object -- An entire collection of tags, preceded by one object tag
that makes up virtually anything in the game. Example: [CREATURE:DWARF]
identifier -- In this instance, it refers to the second half of the object tag, which absolutely must be unique to that object, no matter what.
Objects are indexed in the region files by their ID; nothing more, nothing less. To add, remove, or rename them, one would have to change this index, which, while possible in uncompressed saves, is probably not a good idea.
Entities? (I really have no idea what goes on internally with them. They're quite the oddball of the DF modding system. If I had to guess, I'd assume that it internally stores the values for each individual civilization when they become relevant, which for a vast majority of them will be worldgen itself.)
Tags, with probable exception to entities as usual, are to my knowledge contained only in the raws as of 0.31. Many are used in worldgen, but still not stored, so the effect of a change may be considerably less than the effect of the same modification with a new world, perhaps even to the extent of having no effect at all. Beware.
: I've experimentally made the line referring to tag changes a flat "all". It may well be wrong, but I haven't seen an exception save what's listed, so...
Let me make it perfectly clear to all who read this. If you find anything
that doesn't jive with what I have here, please post in this thread, lest I never hear about it. I can't be everywhere. Also, it's worth noting that I can't test every plausible scenario. I need
you to let me know that I'm wrong. (Excepting the entity stuff - I can't maintain that here. I'll link to a Wiki page where people are attempting to do so if such a thing is made, but I simply don't have the time, patience, or especially the space in this post to try.)