Can I just swap new batteries over in the unit, or does it need reprogramming in any way?

I don't think so. I've had the batteries out my fob for at least a few minutes whilst I swapped them with my spare ones. In fact I just did it right now and I can still open/close the car. If it won't start, I'm walking to the Gym today ;)

I think the code is programmed into a EPROM, so it retains the info even if the power is lost.

Stefan

Burr,
when you say "unit" do you mean the key fob or the back up battery in the alarm system itself ?