I'd continue playing as I don't play Vanilla in the first place.
A is very simple, you just install 2142, update to 1.4, copy the directory, update to 1.5, use the copied directory to play FS. Ok, maybe not very simple, but simple enough.
So basically, you have two bf2142 folders in your /mods folder, and revert between them, the same way we testers have two FS folders (last public release and test version) that we keep changing between?
I think that, judging from the number of times us testers mess up the folders when copying and renaming and patching back and forth, that if we were to revert, it'd be nice if we had a simple launcher application that did the swapping for us. It'd be a very simple program that detected which folder was 1.5 and told you and gave you two options: Launch Vanilla and Launch First Strike. When you make either choice, the app determines which folder in the mods directory has the 1.5 patch and swaps folders accordingly, then launches either BF2142 or FS, depending on the player's choice.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.