by nova
Um...buy a figure...It's really a difficult proposition to try and plot out how to make a custom without having the figures or parts there to work with.
If you held a 25A figure in your hands you'd see that the Waist and Abs portion of any figure are the same part...so there's no easy way to make the swap...at least not what I'd call a beginer friendly way of making the swap.
Look around for a figure diagram that Hasbro was handing out at the last Con...it showed the breakdown of how 25A figures are put together....I've not hacked on any 25A figures to know if there's anything vital inside the ab/waist.
If you don't think you could get by with just swapping thighs and keeping the Ab/waist un-modified, you'll need to get a Razor Saw...also called an Exacto saw or a Snap saw...I got mine made by exacto at Hobby Town USA..one handle...3+ blades...I use it as often as I use my Dremel.
If there's nothing Vital on the inside...simply use your saw to cut both Ab/Waists at the belt line, rearrange, and glue. If there is somthing vital in the way of your blade...you prolly ought to split the ab/waist in a way similar to the upper body...adjust the vital parts so they don't get cut...then saw, rearrange and glue.