My tankless hot water heater went out & is on, I believe, a double 60 amp breaker. My plumber says the new one needs a 50 amp breaker. Can I just swap out the 60 amp with a new double 50 amp breaker?
You can breaker down but not up, so yes, you can just replace the 60 with a 50, the wires should fit. Before you do that though, check with the installation instructions to verify what your plumber wants you to do. The instructions will specify what size breaker you need to have and changing those directions can void any warranties you might have.