The hard part first
If you’re trying to reduce risk, do the hard part first. That way, if it fails, you’ll have minimized your time and effort.
On the other hand, if you’re looking for buy-in and commitment so you can get through the hard part, do it last. People are terrible at ignoring sunk costs, and the early wins and identity shifts that come from the easy successes at the beginning will give you momentum as you go.