My IP has changed and my server was using this IP. I had it claimed but it somehow disappeared. Now I had to add new server which I already claimed, but want to import player stats, etc and more important to redirect users looking for it to the new one. Any help?

Both servers have to appear in Gametracker in order to migrate the server page.
Rank is not maintained when migrating server pages.

If your original server has 'disappeared' then there is nothing to migrate.

both servers are on GT right now. Problem is that I can't claim the older one. Physically its the same server, but I can't claim the older one because IP is different.

Here:
Old one
http://www.gametracker.com/server_info/89.71.193.23:9987/

New one
http://www.gametracker.com/server_info/89.71.145.20:9987/

It is the same server just on GT I had to add new one because I couldn't claim the older one. I also made mistake and choose Portugal instead Poland in the location so I have to wait 2weeks to change it to proper location.

You will have needed to claim the server before it went off line.
The method for claiming the server involves changing the name of the server, which of course, can not be done with the server off line.

Gametracker only knows servers by the IP:port combination.

The Migrate Stats button is actually disabled for TS3 servers because there are no player stats to move.

I have reverted the location flag to the default detected one.

Thank you. Could you also delete the old one or move new in place old so that coming from google users could see the new one??

The server will be automatically remove for extended downtime. There is nothing you should need to do.