I did a name change shortly after joining GoG back in the early days of NMS. I do not recall whether discoveries were affected, though.

I suspect that everything is linked to a GUID unique account identifier and not to the text string used as a player name.

Agree, unique IDs is how it should be, but reality does not always meet expectations.
I did the name change and it's mayuro fka. notanotherone now :)

I still have access to my bases, ships and freighter.
And I hope they appear under my new name to others now.
Previous discoveries (planets, animals,... ) are still displayed with my old name, so Hello Games seems to use only the name and not the GoG UID for this on their servers. New uploads appear under my new name. So no problem.

The only slightly annoying thing is that existing communication modules cannot be changed or removed anymore, beccause they alos run on the old name.

Yes, they do. Your Gogville-base is shown to me as 'mayuro's base' now. :)

It wont be a GUID.

A GUID is 160 bits of crytographically hard random that are supposed to be unique across all computers worldwide. Its also written down in a really funny way, in uppercase hexadecimal with random hyphens thrown in in fixed places, like this: D9A1B59F-E313-4C0C-932B-41E6540C3F96. See also: https://docs.microsoft.com/en-us/windows/win32/api/guiddef/ns-guiddef-guid

For a user id, all you need is that its unique for the database in question. A simple counter sufficies. The first customer gets UID one, the millionth customer gets the UID one million, etc. Much easier.

Agree that a GUID is not needed, but...
/nitpicking on
A GUID (better known as UUID outside the MS world) has 128 bits not 160 bits. And it is NOT cryptographically hard, most versions are not even random. Just a long and complex looking identifier. RFC4122 explicitly discourages use of UUIDs for security purposes.
/nitpicking off

Whow you're right - its even only 128 bit.

I dont know how different implementations handle it. Obviously some official GUIDs havent been created by a random generator. As long as you coordinate that in some way so you dont get collisions (two things get the same number despite being different) thats OK.