Find ways to ship smaller versions of the migration first. If possible: isolate features that can be migrated on their own.
If possible silently run v2 in parallel with v1 for as long as it takes to be comfortable with v2.
Assume that at some point you are going to have to completely halt the migration, go back to v1-only, fix something, and restart the migration.
I'd bet it's going to take 2-3x longer than you think to completely deprecate v1.
Find ways to ship smaller versions of the migration first. If possible: isolate features that can be migrated on their own.
If possible silently run v2 in parallel with v1 for as long as it takes to be comfortable with v2.
Assume that at some point you are going to have to completely halt the migration, go back to v1-only, fix something, and restart the migration.
I'd bet it's going to take 2-3x longer than you think to completely deprecate v1.