Hi, @Chocola.
I'm so sorry for the inconvenience.
As you might already know, the past three servers red, blue, and yellow now became two servers, which are silver and gold.
Red and blue merged together now called silver and yellow just changed its name to gold, so unfortunately you won't be able to merge your accounts to gold, but in order to keep all your previous progress in-game, items, and pokemons, merging is inevitable.
(If you have questions about how to merge check out this link, How To Merge)
There is one way to change your servers after the merge, which is the Server Transfer Request.
Unfortunately, it has recently been closed, so you'll have to wait until PRO staffs open up the service again.
Sadly, there is no set eta for when it will be opened again.
But, when the service is re-opened there will be an announced in both the Announcement Subforums and the announcement channel in our Official PRO Discord Server, so I recommend to stay in touch with the announcements.
I hope this helped you a bit.
Have a nice day.
Kind regards, HappyMango.