When an aircraft is leased out to a subsidiary it should be possible to sell it to the subsidiary, maintaining the schedule. There's a "sell and lease back option" in the asset management, which is basically the other way round. So it shouldn't be too hard to implement.
Agree + selling leased out a/c to other lessee (not sub or owened).
The latter is more complicated since two players are involved, so you need a mutual agreement. Selling to an owned lessee requires just one click.