First of all, yes, a tin lizzie is a long arm. Mine weighs close to 70 pounds. Regarding changing the size of the frame... I agree with Nilla, it's not something that you want to do. It probably is an all day affair. take the machine off, break down the entire frame, take rails off remove bottom support shelf. put it back together, re level it etc.
Why in the world would you want to do that? My frame is the smaller 10' frame and I just did a doll quilt, 18" square.