Can I replace a quad breaker with a tandem breaker?
I would SERIOUSLY reconsider an on demand, tankless electric water heater.   All the hype about the tankless: IE: Why keep 50+ gallons of water hot all the time when you don't use it?   Well, once the water is heated it pretty much stays heated given the quality of the insulation on electric water heaters these days. While tankless gas WH might make some sense (I still don't like them), a tankless electric WH is a MAJOR POWER DRAW and more complicated than a tank type.
I don't think you'll ever get cost recovery on the small energy savings you might get from a tankless.   Have you considered a HP water heater?  I don't know a lot about them, but they operate on a heat pump principle.  I hear their recover time is very slow, but there are real energy savings there.