I would not recommend changing that bike over to a cassette system from the original freewheel. The cost for the components would be expensive, plus you have to spread the stays and if that's done incorrectly you can crack the frame. A 7 speed freewheel is more then enough gears, geez I use to race cat 3 and raced in mountains and never had anything more then a 6 speed cluster (12 speeds total). I still at age 58 don't have more then 14 speeds except on the main touring bike which has 18 and my mountain bikes that have 24.