I think it depends how much time do you have in your hand to start with.
If I want to migrate data without much disruption, then I would use our SnapMirror technology. ( For this you need to install FAS3140 as a independent entity).
Doing this will ensure all the data is there from SAS to FC and would help you take minimum downtime while replacing the head.
No idea on the time taken by 'Disk Replace'.