Tricky, I've not tried this but you can manually change the TPGID number.
Please test well.
1) Switchover to 3PAR2
2) Unexport from 3PAR1
3) Remove from Rcopy Groups
4) Use "setvv -settpgid 1 <vvname>" (sets it back to default for a non PP volume)
5) Rescan hosts, hope they still happy and see the ID change
6) Mirror to 3PAR3 and repeat 1-5 to have the source on 3PAR3 then mirror back
There are also some extra rcopy commands related to permanently switching the volume home from one array to another which might work but not something I've had to play with in many years (and not used since using PP so I've not seen what they do with ID change).