So in the end I used a tool called User Migration Tool. It is designed for upgrading local accounts to domain accounts but worked for my purposes too.http://www.forensit.com/downloads.html
I installed and ran it, selected a profile on the system (from a list provided by the tool) and clicked Assign to. From there I could enter any Domain account. Worked a treat. Quick and simple. No roadbumps or potholes so far.