Nope, just activate the SGSIII on your bionic line. That will kick the bionic off of that line and leave it free to be placed on the line the droid x was on.
The only thing I have noticed is that there may be a bit of a delay for the system to fully update and recognize the phone as being deactivated. However you can check online with the free esn checker, just google it. It shouldn't take over 24 hrs for the bionic to be cleared, and it may be ready instantly. I am not 100% sure on how soon it is available to be activated on a new line, just that another phone i took off of a line took maybe 6-8 hrs to be showing as clear according to the free esn checker website.