Yeah I do not know about any FAR regarding that, but I do know ATC does NOT like it if they operate in the same center they may request a different call sign. If it did get by they send a very nasty letter or phone call to the office. If that situation comes up, I usually change the call sign by removing the final number and replace it with the letter A
I also had a situation where ATC wanted to change the call sign when two call signs were very similar. EX. FLT-5549 and FLT-5559