Just say NO then.
They may be the rights holder, but in not way they could change what was purchased. They could ask not to sell it again but what it is done it is done.
What if they asked you to remove the actual game from the users account?

How come you can conventiently bend ANY of your principle when that suits you?
So after the regional pricing the next thing is EXTRAS-free games?

Also, it's a matter of communication. And that does not cost very much.
An honest company would:
- say no to the rights holder if he require to actual damage your customers. you need both publishers and customers to survive.
A not honest but not behind redemption company (oh wait...) would:
- prior to executing the removal, contact all the users that would be involved in the change.
- offer with no exception refund to all the people involved that doesn't feel pleased by this change. even some kind of compensation for the throuble should be considered.
- Dedicate a news about the matter with the same emphasis of news of the caliber of "Good News..." or "Getting back to our roots" (spoiler alert: you didn't). That at least would give the illusion that you don't actually communicate only when you are trying the sneak behind your customers backs. Also publishers could say nothing if what you post it's true and if by that they (or you) get bad publicity, that's a thing you should live with.

Also I want to point out the fact that while you made pretty clear that GOG cannot be trusted about purchased content (starting the the infamous PR stunt that will always be remembered and brought up occasionally), you also made a majestic job at creating the worst condition for customers to backup their collection.
The actual account page give very undetailed informations (md5?, file date?). You also don't communicate throughly when something in the account changes. You went out of your way to gives files names and folder that is not intuitive and mess up any kind of sorting or archiving method.
For that reasons users have to rely on the wonderful job of people that, without your means and data, manage to do a better job at giving informations that would otherwise not given at all.
Maybe you should consider paying someone to sort out the mess that it's your customer satisfaction policy.

An while having community managers it's good and fun, the fact that 99% of the times they cannot gives proper reply to the users inquiries (because you didn't provide that kind of informations to them) means that you have really great problem to solve about community.
I would like to see this kind of matter treated with the same importance that you usually give when you do a pr news or when you are trying to forge a good image for the company.
There are a lot of critical topic where users get only dismissive and uncertain official opinions when they get a reply at all.