We replaced the heater on our Jacuzzi J335 tub after the OEM heater died. While it's a bit of a process to get the old heater out, there are YouTube videos that detail it very well and a heat gun to loosen up the various hoses is your friend.
Getting the new one in wasn't much harder, just need to be detail oriented to check all the water connections are done up, hose clamps in place, etc., the electrical connections are correct and all grounds in place. We were able to remove & reinstall in probably about two hours of work. Now that we know how to do it, it could probably be done in less than half that.