I think the "Archive & Compliance Management Guide" should be your friend. https://library.netapp.com/ecm/ecm_download_file/ECMP1136381 for 8.1.2, although unless you have specific reason, I would recommend the latest 8.1.3 release for your platform.
SnapMirror will be the answer......SnapMirror between 7.3.x and 8.1.y will work. You would want to make sure your compliance clocks and general times of the controllers are correctly set.
Page 64-onwards talks about this: I would suggest Volume SnapMirror is the *easiest* way to replicate things, but be very aware that with SnapLock *Compliance* (versus Enterprise), you cannot ever 'failback': so when you do your cutover - ie, stop access to live data, update snapmirror, bring your new 8.1.x system online & live - you will NEVER 'failback'. Obviously for a migration this might well be fine !
(I say this because for DR in a SnapLock Compliance environment, you would likely want to use Qtree SnapMirror, which does allow you to do that.....)
Beyond this, I would perhaps add that any 'Compliance' data probably has other access restrictions, etc, and you may have your own processes for proving the data is 'sane' (eg, checksums before and after).
cheers
