Your assumption is correct with regard reinstall XP
The O/S sees new MoBo and assumes new computer- MS will need to re validate it.
From then on normal transfer of data.
What about programs?
Does Acronis hold a complete back up?
From my experience with attempts to transfer programs- it does not work - because, seemingly vital elements in the registry of the deleted O/S have gone too - so a re install from disk for programs is the only way - or has been so for me.