Reversion sucks since you have to delete ALL snapshots for ALL volumes prior to reversion. This is a requirement to revert / downgrade.
I just went through a similar process two weeks ago to convert a 32-bit aggregate to 64-bit to utilize SRM 5.0. Wasn't the most fun I'd ever had...but it worked. I get to do it again in April (before 8.1 GA but alas, I'm using 3210's for this and cannot leave 8.1 on there even if I wanted to since they have FlashCache).
But I assume you want to upgrade the 6240 to 8.1, correct? And then just use the new VSM feature to replicate as your migration strategy? I would hope the aggregate(s) for the 6240 are already at 64-bits, however, even before any data is put on there and regardless of ONTAP 8.0.2 vs 8.1.
What's the current replication / off-site strategy for the 6040?