My forum name is user8945, but my actual Minecraft game name is snoopy8945. Can that be updated in my Sandlot account so I can access the game?
Your forums name updates when you join the lobby of the sandlot. Try logging in and seeing if that syncs them.
These issues (forums name and game access) are unrelated. Your forum name does not affect your ability to join in-game. I will send you a conversation about this matter.