I had an eight year old Weber propane grill. When I redid my kitchen I
looked into converting the grill to NG. The cost was higher than I
expected so I simply bought another grill (a Weber Silver B NG).

I now have both. First of all, the Silver B grill, while not as shiny
as new, still looks good (the older grill is red).

The only parts I've replaced on the grills (and they get a lot of use)
are the grates. You'll be replacing the grates either way, so it's not
a big deal compared to convenience when you purchase the grill. I
think, though, that the porcelain grates may last a bit longer.

My original grill had cast iron grates which I really liked (they took
a bit longer to heat but they held the heat much better) but they did
eventually corrode and had to be replaced.

One thing I have noticed, though, is that the NG grill can get MUCH
hotter than the propane grill. It may be the fuel or perhaps there was
a design change over the years between manufacture dates (Weber says
the fuel should make no difference in temperature).

As far as moving-- converting back to propane would probably not be
worth the trouble vs. getting a new grill. And when you go to sell
your house, having a NG grill may be a selling point with the
purchaser- you never know what might get someone's interest (what sold
my old car, quite a number of years ago, is that it was the only used
car for a thousand dollars the buyer could find that had a tape deck).

Best of luck.

Shaun Eli
http://www.BrainChampagne.com
Brain Champagne: Clever Comedy for the Smart Mind (sm)