When they come out with plug-in hybrid cars, it's almost certain to involve major redesigns to the chassis – they have to make room for the extra equipment, notably the battery pack. It's conceivable they might design a chassis that's ready to be either gas or plug-in hybrid – but my intuition and experience says that's just not likely.

Now, let's talk about powertrain swaps for a bit, since that's what you're thinking about. Your intuition is correct, these things *can* be unbolted and swapped around. Lots of people do them. And the same basic principles apply, even if you're going from gas to hybrid as you wish to.

It's really the easiest if you stay within the same chassis. For instance a '92 Camaro engine in an '82 Firebird – these are the exact same chassis so everything just bolts up (and often just plugs in). However if you change chassis, say, put a Prius engine in a New Beetle, or even a '09 Prius engine in an '01 Prius… things get harder. This can range from "a little harder" to "really, really hard, what was I thinking?"

Be clear on this. Nobody does a powertrain swap to save money. They do it as a hobby – as a labor of love. You could not pay someone to do this, it would just cost way too much.

Besides, there's no such thing as a "car improvement loan". Never attempt a swap on a car you owe money on, because if the lender finds out you've "ruined" their collateral, they'll call the loan (demand all the money now).