There are some good articles on the subject, really it doesn't seem that complicated, I've considered converting my fixed mag versions to detachable over the years, using my D-Box as a template.
At the end of the day though, to be quite honest, I can reload the fixed mag versions faster with stripper clips than I can swap mags in the D-Box. And the fixed mags don't catch on regen in the bush (try hiking through tight immature spruce with anything extra hanging off). So I didn't bother. Also, AK mags are pricey and not that readily available anymore (Marstar does have them and at a good price). Only my two cents on that - it's like swapping a small block chevy into an S-10, sometimes you just gotta try it.
If I have time later I'll pull the trigger groups out of my D-Box & fixed mag models and confirm but I believe you're right, the groups are the same.