Exchange 2010 to 2013 Migration

OK, looking at migrating exchange 2010 to 2013,

Have setup and installed 2013 and moved a single mailbox to the new server.

When we open Outlook 2013 for that related mailbox we get a password prompt, when I checkout the Outlook 'Connection Status' Window it seems the password box is the Outlook connecting back to the old 2010 server. (does not happen for users with mailboxes on 2010 server)

I'm wondering if its trying to connect to public folders held on the 2010 server, as I cannot view public folders without entering the password.

From what I understand moving public folders to 2013 is a one time deal, you move and you don't move back, whereas before in migrations I have always replicated, tested and then finally removed the old one. this means that I can't test my theory until I move all mailboxes to the 2013 server, piss everyone off with password prompts, and finally move the public folders and hope all works OK.

1) Am I missing something?
2) Is there a better way?
3) Can we solve the password prompt thing and not annoy everyone? (ticking remember does nothing)
4) Current thought it to 'Disk to VHD' the 2010 server, capture the 2013 VHD as it is and then move both to a test environment, spin up and give it a go, but this seems a little excessive. Thoughts?