Just move the tape to another pool and continue on please.
It looks easy on the surface, but if go deep into details, we will see there many unknown variables.
Say, while moving media to a different media pool should only one media or the whole media set be moved, should a media inherit retention settings from a new media pool, if it should, than it should become a part of one of media sets existing in new media pool, as retention setting apply to media sets, rather than to individual tape
While we're at it, be able to continue writing to an imported tape as well please rather than it becoming R/O.
I'm not sure whether this will be possible. Such feature would require tape ability to host two different mediasets with different retention settings, which might be tricky.