Exchange 2007 (Active-Sync) mailbox profiles which are being used from Exchange 2007 might be required to be re-configured on Mobiles devices upon mailboxes being migrated to Exchange 2010.
This statement is applicable for certain version of mailboxes but not all. Kindly work on your end to explore above statement and send your inputs to list the impact for client Mobile users at the earliest possible Current version of devices at client is listed below.
Vast majority are Apple users -iPhone and iPad (with run Apple's IOS). Then there are quite a few Android users (versions cover the spectrum, but the majority are probably Android 2.2). Finally we have 2 Windows Phone 7 units