Is there a way to assign tapes to a different VTAPE group AFTER the tapes have been created via a command/utility or perhaps during the recycle process?
No, there is no utility for changing the group assigned to a virtual volume.
The group is assigned to the virtual volume when it is first mounted as scratch. Once assigned, it cannot be changed until the volume is subsequently scratched and then remounted.
Even if the data set and group filters are changed, then the group number assigned to existing virtual volumes is not modified.
The only way to change the group is to copy data sets to new volumes.