Setting AEA (Associated External Accounts) on the resource Exchange forest when migrating account (4217700)

There are 3 different domains each in its own forest:
A - contains disabled AD accounts holding Exchange mailboxes. AEA (Associated External Accounts) are set to the corresponding users in B domain.
B - accounts domain containing AD accounts where users are logging into.
C - target domain.

B is migrated to C. Target users will need to continue using Exchange mailboxes residing in A resource domain. How can you flip AEA (Associated External Accounts) to correspond migrated accounts from the domain C.