So trying a few things, mainly from here, https://aptitude.alioth.debian.org/doc/en/ch02s05s01.html#secDisplayFormat I wasn't able to get an error-free change to aptitude that displayed the repo of the existing package and the updated one, which is what I would like.
(I know about apt-cache policy, show, info, etc. but would like to see in aptitude --full-resolver these results).
I added "origin" into "default grouping method" and a "%O" in the "display format for package views" and it did display a repo name, like "Testing", but I wasn't sure how to get them both in. Also everytime I start it it complains of an error - unknown grouping policy "origin".
I'm using Debian.