We've recently installed a 3340 appliance along with our existing 5240.
I'm currently moving monthly backups between the 5240 and the 3340 using bpduplicate and setting the rl to 9.
Current monthly SLP points to 5240 and is infinite.
How do I expire the backups on the 5240 and retain the copies on the 3340?
Create new monthly SLP point it to 3340, then change the old monthly SLP to expire the backups on the 5240?
You can confirm that there is more than one copy for images using commands like bpimagelist or images on disk report.
Save the image id's where you have confirmed copy 1 and copy 2.
Use bpexpdate to expire '-copy 1' of image-id (-backupid).
(Command usage in Command Ref Guide)
Do not be tempted to use the 3340 as the first backup target - the performance of the Access appliance is not designed for that purpose and should only be used as a duplication target.
Other than that adjust the SLP as required. You shouldn't need to wait for the images on the 5240 to expire before you change the SLP - if an SLP is in progress it will continue using the original SLP destinations (NB it is possible to change this behaviour if desired).
My suggestion would be to adjust the SLP something like this:
Backup -> 5240 MSDP - retention 4 weeks
Duplication -> 3340 - retention infinite (do you really need to keep that backups forever?).
Adjust the retention for the original backup to satisfy typical restore requests, so that the majority of your restores would come off the 5240 (which has a better performance than the 3340).