Hi!
You cannot change prefix on the existing system. If you go to RSA1 and check your backend BPC objects, you will see that each cube and each infoobject has that prefix - /CPMB/xx, where xx is your prefix, hence it cannot be changed.
The second option will work and the transports should be fine as long as you don't forget to check box "Use technical names from backup" in UJBR, this way the prefix will stay D6 and won't change.