Yep, everything in my house is a retrofit unit…. Tall skinny water heater in a tiny closet, air handler in the attic…
My water heater used to be in my cellar, which is dirt floor, and sucks to get to. I got a scratch and dent 80 gallon water heater and move it to my garage and ran the plumbing out there (its an attached garage).
Amazingly, my wife can still use all the hot water up with an 80 gallon tank.