How to return to the vanilla version?

[DN308]

I try to go to the 4-15 version but I can’t.

 I tried another install of the sim (Forgotten Battles) on another disk and this one works.

 I have the Ultimate Edition installed on my main hard disk but I would like to return to the original version and try the process again up to the 4.15.

Is it possible to do it without deleting the whole game? I can’t find where the original game disk is….