Can they be changed on the fly? I want to changed the IDENTIFIER and KEY on my BES 4.0 server, but I don't want to have to reinstall the software. If I do it, will it work?

You can do anything.....however there are consquences for every action.
Changing the SRP settings would imply that you are changing your BES either from Temp SRP settings to Permanent SRP (these are the SRP ID & Auth Key you would use on your production server). It'll work....but if this is a production server, your users will have to recradle or do an EA. Don't forget about the CALs as well.