Adjust migration to handle data changing part specially

I can't recreate the original issue but, according to a comment by
@vdboor, this change should ensure that the date migration does not
conflict with the schema migration.

Going forwards, I will ensure migrations don't mix data and schema
changes but at this point, it seems better to adjust this migration
rather than renumber (which causes problems for anyone who has already
applied this migration).

Fixes #977 (hopefully)