This is not possible using migrations. Migrations only use the source subscription's and destination plan's information. There is no intermediate state draft subscription that would allow changes to take place. Changes must occur either beforehand and be carried across into the new plan or afterward and update the resulting new subscription.
To perform a similar behaviour to a scheduled migration, which allows for changes to be made to the destination subscription:
- Set the current subscription to expire in 0 periods. This will have it expire at the end of the current billing period and complete any end-of-period charges.
- Set the new subscription to a scheduled activation date on the same day as the old subscription's recharge date. Make sure that this is only set to provisioning.