The Rails 4 Way page 177:
Sebastian says…
A little-known fact is that you can remove old migration files (while
still keeping newer ones) to keep thedb/migratefolder to a
manageable size. You can move the older migrations to a
db/archived_migrationsfolder or something like that. Once you do trim
the size of your migrations folder, use therake db:resettask to
(re-)create your database fromdb/schema.rband load the seeds into
your current environment.