Correct, you're not performing a cross platform dump and load (XPDL), so no need for sp_post_xpload.
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As for the cutover ... assuming you're not using Replication server ... and assuming you're not performing non-logged operations in your dbs:
- dump and load full database dumps; can be done while users are still active in current dbs
- start performing tran log dumps and loads to keep new dbs in sync; can still have users active in current dbs
- when ready for the cutover, kill off all user/application activity in the current dbs (I'd vote for bringing up the current dataserver on different ports - either bounce ASE, or use sp_listener to startup new ports and shutdown old ports - so as to make 100% sure no users/applications can modify the current dbs), perform a final tran log dump and load to bring over any last minute txns from the current dbs, then let users/applications into the new dbs; downtime should be limited to how long it takes to perform port switch of current dataserver, perform last tran log dumps/loads, and point users/applications at new dbs