water heaters come with 6 9 or 12 yr warranties. best bet is to call the plumbing company that did your house. they will be the better ones to call. if they are a large company they may be able to do the swap out and just charge you labor permit and upgrade. remmebr the only thing covered is your water heater. as for a metal tag not looking in the right spot. your heater should have a white labe about the size of an index card or bigger. your model number and serial number is on there. you also have a 1800 number on there you can call them and they will be able to get you with a warranty authorized plumbing compnay.

I KNOW cause i am a master plumber and deal with warranty swap outs all the time.