I did some mods on a 20" frame for my son earlier this year, and just started on mine yesterday. I used a hacksaw to cut where the cantilever tubes were welded to the top tube and the down tube and heated the chainstays to drop the rear height. Next job is to cut off the head tube and give it some rake.
20" frame
The start of the 26" frame
20" frame
The start of the 26" frame