The plumber is a world class con artist, I wouldn't let the guy set foot in the house again. If you have a P&T valve that IS NOT leaking, your best bet is to not go near it. They are a very simple device, and will work just fine, decades after they are installed, as long as they are not screwed with. If you test it by flipping the handle, you have about a 90% chance that it will not seat correctly, and will leak until you replace it. As slackmax correctly notes, they can be an absolute bitch to replace.
The other scheme that getting some play in my area is plumbers and home inspectors (another group with a high percentage of scum sucking con-men) are telling folks that they need to replace drainage piping on P&T valves since it's not soldiered copper pipe, and is a serious safety issue. Bullshit. The current IRC code currently has a long list of acceptable piping for this application, and it pretty much condenses to, "if it's legal for use in water supply lines, it's fine for P&T drains" CPVC, PEX, copper, etc......