How many entity files are we talking here? If you put all of your brushes in custom windows files and didn't mess with the vanilla ones too much, most of the art assets won't need changing. Entity files have changed a lot though, since there's been two expansions and dozens of updates. It has been done for very large mods, but depending on how many entities there are, as Harpo said it may be better to just redo them with a Rebellion base.

If you search the modding forum I've made a few threads over the years listing all the changes with major patches. They might help you narrow down some things.

Yes, you pretty much have to re-do your entire mod from scratch, and yes, that is how the huge mods have done it, and yes it is a massive pain.

I did look into modifying the mod updater to do this, BUT the shear number of changes(both additions AND deletions (and also moving sections of the entity around)) in each of the types of enties made it almost impossible to automate.

THAT is WHY I said it was a manual task.

harpo, the NON-subscribing ghost