I can confirm that what Grimlocke says, is true. I have modded many reactions and buildings in before and have forgotten to add them to the entity, or just mistyped the name. I've also successfully removed reactions and buildings from the entity file. Not to bash on Deon's work, but I thought there were too many buildings for skill leveling and that they were getting too complex. /shrugs
I'll say it here, just as I've said it other places.
Adding new reactions, buildings, creatures, etc. require a new world. Regardless. If you add a new entry and try to have it show up in your already generated world, it's not happening.
Editing existing reactions, buildings, creatures, etc. mostly do not require a new world. Depends how extensive you're getting with your modding... I don't think you can add a new caste, but maybe. Also, you it is possible you won't see any of your changes to present dwarfs. Like if you mod dwarfs to be super hot and fire immune, you'll have to wait for your next migrant wave. (fire immune may be changeable on existing dwarfs) So those new migrants will be super hot, start forest fires, burn away water, warm stone, turn any items in their tile and one tile away from them into ash, etc. You can then mod it again for dwarfs to be extremely cold and have no cold damage point and they'll freeze anything that gets close to them.
Again, my motto is, 'If you don't know if it needs a new world, just gen a new world.' Also, just try it! Seriously near everything I know about DF is from me modding endlessly and figuring stuff out. I've probably logged more test hours than actual playing hours.