Hi there,
Unfortunately, for now, you can't change your server for now because the Server Transfer Request has been closed for a while now. You will have to wait until PRO Staff open up the possibility to players again and there is no eta for it. Just keep on a lookout for the Announcements subforum or the announcement channel of our Discord Server. You can opt to play on the Silver server with your account from Blue or Red server and to do that, you will need to merge your account.
I've checked and you have successfully merged your account. Should anyone have any other issue with the merge, please do check the Common Merge Errors: FAQ.
Since you have not been around for awhile, some things have changed. For your information, Red and Blue is now the Silver server while the Gold server is formerly known as Yellow. Even if you played on just one of either Red or Blue server, you will have to merge your account or it will say "invalid username or password". You can find the details about the merge by checking the Merge F.A.Q.
As your issue has been resolved, I'm going to lock this topic now. Thank you @Zupercooer for your fast response. Have a great day everyone!
Best regards,
Astreus.