[rockfan4]

I guess the two questions I have are -
How old is the deck?
How old is the water heater?

If it was me, and I'm going to live there a while, I'd wait until it's time to redo the deck, and schedule the work while the top boards are off the deck. Hopefully the water heater lasts until that time, as I'd hate to have to replace the water heater before replacing the deck. I guess if you did, make sure it's a model that can be converted from LP to Natural Gas.

I just talked to my brother-in-law, and he was lamenting replacing his deck. He put it on in the 70's when he built the house, and found out he's not as spry as he was 50 years ago.

Edit: I thought about it some more. How high off the ground is the deck? Can a guy get under it, let's say, to trench the gas line? If it is, you could cut an access hatch in the deck (look it up on youtube) and then they could get in and install the meter, and you wouldn't have to tear out the whole deck. They'll probably complain they can't run their trenching machine under the deck and refuse to dig it by hand, but I don't really see another option.