From Centos 6 to Centos 7 with a big account

Hello, Please tell me how to solve the problem, there is a server on Centos 6, it contains only 1 account, but a very large one (1.5 TB of data / 7500 mailboxes). I would like to upgrade it to Centos 7, but how can I do that? It is impossible to update the OS on the server, I think it is also impossible to make a transfer to another server, because it will copy forever. How can this problem be solved? Thank you.