Upgrading to APG needs rethink of the "Engine" argument

[robins]

Finding upgrade paths for Aurora Postgres needs some rework on the "Engine" argument, since that changes when migrating from Postgres to Aurora Postgres.

Similar needs are probably there for MySQL -> Aurora MySQL too.

[robins]

Seems difficult, since find a possibility to upgrade to APG (or AM) requires providing the ARN of the source Instance, which may need re-thinking as to whether this is even possible at this level.