When the beta for vray 3 was available I made the switch during a project. But I had a longer work relation with the client so I knew if something would break I could talk it over. Overall I didn’t have a single problem related to the job and everything went well. But having said that I agree with Vlado it is probably not the best idea to do this mid project because stuff can break even if it shouldn’t.
If you decide to make the switch though, make sure you save an version of your project that is working flawlessly with your current installed vray version. So you at least have a backup you know that works.