You have to get ALL the water based coolant out of the system.
That means you need to flush the entire system, including the heater core. If you don’t do that you are just pissing away money.
IIRC Evan’s has a chemical that you use when the engine has been run with water based coolant. Use it.
If the Evan’s coolant is building pressure there is residual coolant left in the system. Make sure you get it out.
I’d never go back to conventional coolant.