Exchange Cross-Forest migration and Autodiscover

We're planning a cross-forest migration for a company splitting scenario.

At the moment there's an exchange organization with several accepted domains which the two most important are domainA.com and DomainB.com

The migration will involve DomainB.com users.

During the migration we will have a coexistence period where domainB.com users are on both the AD domains.

The migration will be carried with ADMT tool.

My question is about the autodiscover URL which is now pointing to the current Exchange infrastructure.

When an user is migrated with his mailbox and PC his outlook client, and the next startup, will query the autodiscover pointing to the "old" exchange server and, I think, it will fail.

Can anyone share any experiences on how to carry this kind of migration with the least impact on the users ?