hey. i'm no serious mountain biker, just the usual city stuff. becusae i'm scared of breaking the bike, i have a very old one. it's a Raleigh (all sigh) but it's good. i like the size of it and i have all 18 gears (may think about adding some in the future). my main query is, can i change the wheels on it? i noticed today that it has smaller wheels then my other mountain bike. (also old). it has 24". now i don't know if this is why i prefer this bike over the other one, but can i fit 26" on the same frame? and what difference will it make to my ride?