Run v14 and v15 side by side on your computer
 

If you have secure PBEM games running on SPww2 v14 that you need to complete but you also want to start using v15, here's a way. It's a bit of a juggling act but you may think it's worth it.

Copy the "C:Shrapnel Games" folder and subfolders (default installation on my computer, yours may be different) to another directory called “Shrapnel Games2” (different harddrive, same harddrive, makes no difference).

Upgrade the new directory to SPww2 v15. Retain SPww2 v14 on the original directory.

I tested the new v15 installation by temporarily renaming the original installation folder to “C:\Shrapnel Games\The Camo Workshop\WinSPWW2 xxx” so the new installation couldn’t access any files on the original directories. Ran the new v15 executable. Worked fine. You can skip this step if you want.

Now you can continue v14 secure PBEM games on the original installation. You can start new games, both basic and secure, on the copied installation.
You can’t move any secure PBEM games from the original installation to the new copied installation directory but that's not the intent anyway. After you finish all the v14 games, you can upgrade your original installation to v15 and delete the copy.

Keep in mind that if you start new secure PBEM games in the copied v15 directory you won't be able to move them back to the original directory so you'll have to keep your copy around until you finish any v15 secure games you started there. Basic security games can be copied back to the original directory.

This should also work for the SPMBT upgrade but I didn't try it.