First off, if you are wanting to have a period of no downtime, then don't expect that. I would go ahead and set your superiors' expectations to have downtime (although not much is needed).
I do not believe there is a way to do what you are wanting (BES account in AD; access to 5.5). I could be wrong. I'm not sure what other people did in their migrations, but I simply built up a new server (with test SRP) on the new AD with a new service account and then when we migrated users, they were added to the new server. The old server was decommissioned and the SRP information was changed on the new server. This step requires the end-users to recradle twice (once after their account migration; once after the SRP change).
I had one issue after that with the SRP not being updated for MDS (even though I did it within the management console). I had to drop the database and recreate it (5 minutes worth of work - no data loss or post-work user impact).
Anyone else have some best practices for Domain migrations and Exchange upgrades?
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