After changing my nickname, I lost all my items. After contacting the server support, I received information that there was nothing they could do about it. I contacted Minecraft support and they wrote that I may have experienced bug and should report here. It's also interesting that on some websites about Minecraft players finder, some use my new nickname and others my old one. All websites (whether those that use the new nickname or the old one) they give the same uuid. After entering uuid instead of the nickname, the new nickname appears on the websites that worked with the new nickname, and on those that they work with the old nickname - old nickname is shown. If a website has such a thing as a history of nicknames, the ones that show the new nickname have the old one in the history and the ones that show the old nickname have nothing in the history. When I join the server using, for example, T-launcher and entering my old nickname, it says "Not authenticated with Minecraft.net" or something like that. The server supports non-premium players and when I join it from the official launcher, it detects as if I were a non-premium player.