I have to ask: If you are using WLM 2012 strictly as a POP or IMAP access e-mail client why would you have to change?
There are no indicators that WLM 2012 has become a security risk and the POP and IMAP protocols have been virtually untouched for decades now. Until something comes out indicating that a new exploit (or exploits) are targeting WLM 2012 successfully it's probably as secure as most of the currently available e-mail clients. Official support only stopped in January.
If you are using IMAP access you don't need to save anything (unless you moved it to a strictly local folder) as IMAP folders are on the mail server and will be re-created when you enter your account information in an alternate e-mail client if you wish to use one.
My first choice is Thunderbird. Opera Mail is quite nice, too.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
~ Brian Vogel