Hi, I have the old Mk1 birdy with 3speed hub with 7speed/cassette gears on the back wheel.

is it easy to just change the “cassette” 7speed gears with a 8speed or 9speed cassette?

and then prob would have to change the grip shift gear selector on my handle bar also to a 8-9speed setting one.

as I’m after a couple more slower/lower gears to save my knees from surgery in the future

any help would be appreciated