So many replies, I need to check them more often!
Yes I did use the "-recalculate -ret level" option.
This modified the expiration date of the images and the expiration date of the media, but not the corresponding Ret Level, which stayed at the value corresponding the old retention.
The retention level doesn't really matter, once the exire time has been changed by bpexpdate.
The time it would matter, is if the images were expired, and then imported - the expire time would revert back to the time set for the level number, relative to when the import was run.
Do not ever mix retentions on tapes, short term it may help, long term you will use a lot more media.
What you could do is reduce the number of partially full media for the volume pools to the same number, as the number of consecutive write drives in the storage unit(s). For example, if you had 5 drives, set this value to 5 (in the volume pool) - this means that no more than 5 tapes are ever allowed to be partially full, meaning that you would have to fill a tape before another could be used. Having 5, means that all tape drives can be used at once.
It is not 100% guaranteed, there are times you may use a few more tapes than 5, but it should reduce the number of tapes used on average.