If you’re referring to the altitude transducer, as I think you are, it’s an expensive proposition and a lot more a potential issue...for the 200, not the 150. No where near. Reason is the way the alt transducer couples to the static system in the 150 is different from the 200. I bet you the beers this can be confirmed with autopilot repair shops if you ask them when was the last time they saw a 150 alt transducer go bad. The answer may be next to never!
The 150 is a very capable, solid and proven autopilot. With the exception of giving it digital attitude reference by removing it’s vacuum dependancy there is nothing else pressing that can improve on it.