This is based on the flexvol understanding but I am guessing (not 100% sure) it should work with flex-group as well.
The key switch to use when deleting relationship is to use use: '-relationship-info-only true' [With this switch : The cleanup of Snapshot copies is bypassed and only the source relationship information is removed.]
::> snapmirror release -destination-path svm_name:volume_name -relationship-info-only true
In Short: [You can do this by GUI as well)
1) Go to relationship tab.
2) Break it
3) Delete it, first check-box 'ok' to delete relationship (select that) but don't select another check-box 'ok to release base snapshot blah blah. This is equivalent to '-relationship-info-only true'. In other words, we want to keep the 'base-line snap' memory intact. This will allow you to have the destination volume data as well as all snaps intact.
4) Create a new relationship with destination and carry on from there.
If incase you want me to reverse then,
5) delete the new existing relationship ONLY.
6) Re-create the relationship between original source and original destination.
7) Go to GUI again, relationship tab - You should see the relationship you created in step 6.
From the GUI option : Select 'resync', this will re-enable the mirroring without baselining all the data.
This one has info on reversing or but you should be able to do regular forward resync: