Are there changelogs somewhere I don't know about that detail what changes are made when an installer is updated from 1.x to 2.x? I have a ton of GOGs that were "upgraded" perhaps just to Win8 compatibility (maybe? and if so how?) but I see no documentation anywhere about them. It would be nice to know what changed.

I thought I saw something about the changes not being such a great thing for certain games (Infinity Engine games maybe on certain systems?) but I can't recall now. So far I've been inclined to keep the 1.x installers also but this is taking up a lot of space.

I just perused the forums for Baldur's Gate series and PST and don't see anything about issues there so I must be wrong about the above. Anyway, it would be nice to know what all those new installers were for and what they added in specific terms on a game by game basis.

Why not a sticky thread in each of the game's forums or a link on the shelf to communicate changes?

This seems a reasonable enough thing to ask for. The Wing Commander IV installer is over 6 gigs in total to upgrade to 2.x but I could not find any information about it in the game's forum.

One last thing, are you guys saving your 1.x installers "just in case?"

There is a "What has been updated" thread both on GOG.com and on GOGWiki, though I'm not sure if both are updated as often as they should. Most of the 1.x to 2.x installers are just for completeness sake, and should be safe to ignore (though a few also include updated DOSBox/ScummVM installers).
As for keeping the older installers, I do for the moment, since I want to install them first, grab the rectangle icons, then archive the icons somewhere.

Hmm, I think I will play it safe (maybe I have learned something!) and keep EVERYTHING! lol

Mine! Mine! These files are ALL MINE!!