When migrating a subscription, can I make changes to the new subscription before completing the migration?

This is not possible using migrations. Migrations have only use the source subscription's information and the destination plan's information. There is no intermediate state draft subscription that would allow changes to take place. Changes need to take place either beforehand and be carried across into the new plan, or afterward and update the resulting new subscription. 

To perform a similar behavior 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.