Plumber wants to replace PRV valve into house at same time he is replacing hot water heater
Normal household water pressure is about 40 to 50 PSI. A water heater pressure relief valve is normally set to to open at 150 PSI so 90 PSI voiding a warranty seems like a stretch. If in fact your pressure is at 90 PSI I'd be concerned about your faucets and appliances valves, ice makers, etc.. because 90 is too high.  I'd ask him to check it again and watch him do it (so you'll know how to do it in the future, LOL) if it's high, you might want him to do it. If you have him do it, think about also having him replace your main shut off as those almost always go bad and they're usually right next to the PRV. Good luck and stay safe out there.