Help me, please.

I really did try to figure this out myself, but failed.

What is the difference between a double and a compact double?

Say (hypothetically) one bought a Waterford with a double, which from the picture has very little difference between big ring and little. And you KNOW you need a littler little ring! Maybe even a littler big ring, too.

Couldn't you keep the cranks and front derailleur and shifter and just change out a ring (or two)? (and adjust things afterwards, of course)

Someone told me I'd need to get a compact double and change out the cranks, too. And the FD.

Isn't a compact double just 2 rings with a big size difference? Can I fake it with new smaller ring(s) in front?

Switching out the double for a triple would cost me a gazillion dollars, and honestly I hardly ever use my big ring on my commutermobile. Don't use the little ring much, either, except on monster hills.