I would recommend decrypting the Laptops before migrating as the encryption keys will be held on the 4.5 server.
Generally the best way to do the test is
- Build 4.6 as you want it will all software and policies
- Stop any automated deployment tasks on the test systems on 4.5
- Decrypt the systems so EEPC does not have any issues
- Manually force an install of the 4.6 agent and they will check in
Perform your tests and then either start the migration or roll back to 4.5
EEPC adds a level of complexity and I am unsure how this will act in a migration scenario, and an upgrade from 4.5 to 4.6 may be a better option if you have thousands of systems to decrypt and re-encrypt