I have done so on my hyrdo spa which is 13 years old
It is not rocket science, and easy to do if you are reasonably handy.
What you need to do is pick up a spa-pac complete with control panel, heater and assembly.
You do need at least a basic knowledge of Electricity and safety, because you have to wire straight to the pac.
Ez-peazy.
Worst thing about it is paying the bucks.
About 550/600, but you get new control panel, heater, board, etc. beats buying a new spa.