1 - Why not merge the the versions so one change affects all versions.
Use case statements to run the appropriate class/form/procedure etc.
I worked on a warehouse logistics package where my first priority was to
merge 3 versions.

2 - Another option is to move all the identical classes, forms etc into shared directories which every project can access. Therefore changing code here will affect every version.

3 - Rename all dissimilar classes, forms etc. I'm guessing you have identical names for classes, forms etc in each project. If you rename each class, form etc with the base name + Company ID then you could esentially use the same code to run different classes, forms etc

EX: Employee screen name = employ. Change this to employIBM
So when IBM person runs your program you call do form forms\employ+"COID"
where COID ="IBM"
Downside: Your exe's would contain code for all versions so would be significantly larger.

- Your call