Hello.

[Mad Mike]

you have to save as a different name your v1.00 .exe file in able to continue your old PBEMs. while playing old PBEMs , you have to execute v.1.00 exe but for new PBEMs v1.01 exe. I mean you will have two .exe files

I think the safest way I have heard of yet was to copy the complete game directory (old) to some other location and then patch the game to the next higher version. You could then launch the two versions as required.
Just renaming the exe might work, but I think in most cases also some other content is updated, resulting in an acompanying .brz container file. This would definitely cause confusion if just the exe was renamed. Hence the suggestion to have two completely independent directories with all the necessary content.
At least that's what I gathered over the years from CMSF and comments about different versions. Never tried this myself though (I only started PBEM with CMBN).