The lfs_migrate tool does not currently preserve the PFL/FLR/DoM components of the layout when a file is migrated. By default, it will get the stripe_count and stripe_size of the file (maybe pool and mirror_count if https://review.whamcloud.com/32977 is updated to do so), but it does not preserve more complex layout details.
Dropping the initial components of a PFL file is probably desirable, since it is unlikely that a file will become smaller, and it makes sense to keep the stripe_count of a file at the widest needed.
We do need to keep the number of mirrors across migration, and it would probably be good to keep any OST pools per mirror, if they exist.
Separately, it would be useful to have some way to migrate only the mirror(s) that exist on particular OSTs if we are trying to empty particular OSTs.