Do yourself a favor and drop the money on the Simms pant wader, if you're committed to a pant. Going to breathe better, last longer and better warranty then anything else on the market. If your buying pant waders form comfort, get the most comfortable on the market..
If you want a convertible, just get the regular G3s. the regular G3s can convert by rolling down the top and using the shoulder clips as a belt. Plus you wont lose the nice features like the hand warming pocket and inside flip out pocket. And going from waist to chest high or vice versa is quicker and fool proof.
Oh and go to a shop that has people that actually have experience in wearing and selling waders, not somewhere where they're sold beside the snowshoes...