My house is wired 12/2 and 20 amp breakers. I bought a new water heater with 2 4500 watt elements. Did not realize I needed #10 wire and 30 amp breaker. To not replace wire and breaker will I be able to change elements out from 4500 to 3800 watt? If this will work it would be a lot cheaper. The water heater I am taking out has 3800 watt element in it.

Your current heater is 240 with a 12/2 and 3800 watt elements?.

It should be no problem. It just will not heat as fast.