Having worked the plumbing trade from along the entire Gulf Coast from Florida to S.Texas I can testify that water heaters are commonly installed in the attic with complete success, HOWEVER:

The attic mounted water heaters are generally placed there during initial construction of the structure and all the necessary provisions for them are planned well ahead.

Retro fitting a water heater to the attic can be a very expensive undertaking.

First off, let us consider the code requirements for an attic installation:

1. Under the IRC the access hatch to the attic MUST BE a minimum of 22" x 30" while the UPC requires 30" x 30" min.

2. The physical location of the water heater may be no more than 24' from the hatch.

3. You must have a 24" wide solid flooring catwalk from the hatch to the WH location.

4. There must be a finished 24" solid flooring walkway all the way around the water heater.

5. If you use Type B flue pipe you must have a minimum of 5' vertical clearance from the top of the WH to the roof decking.

6. There must be an electric light fixture in the near proximity of the WH location and the switch to control that light must be located within arms reach of the attic access hatch.

7. There must be an electrical service outlet in the near proximity to perment servce techs to use electrically operated tools while servicing the WH.

8. The WH must be equiped with an approved drip pan and the drip pan must have a minimum of a 3/4" drain line to an approved discharge location.

9. The WH T&P valve must be fitted with an approved "full bore" drain line equal to the diameter of the T&P discharge port or 3/4", whichever is greater, and the line must run to an approved termination point.

NOTE: The T&P MAY NOT discharge into the pan, and the T&P and Pan drain lines may not be combined.

10. After you get all of that done, you still have the expense of getting the gas line to the attic location

Now as was mentioned above, you must also consider the weight, but not just the weight of the water either.

Empty water heater........ 200/bs +
Water equals 7.43lbs/gal so a 40gal would contain 297lbs of water while a 60gal WH would contain 445lbs of water.

You then have to consider two servicemen @ 250lb each plus their tools, which would probably be another 100lbs.

That works out to approximately 1/2 ton, and you still have to consider the weight of the floor decking.

As you can see, that would not be a problem in new construction when the trusses are initially designed for the load, but you can be sure that would exceed the design load for existing trusses.

Now in regards to moving your existing WH. Studies have concluded that the average life of a WH tank is 12 yrs (Which explains why the mfg are so intent on selling you a 6yr warranty...LOL). Your tank is possibly 20 yrs old, so it has definitely outperformed its designed limits. Left undisturbed it might go another 10 years or more, but if your luck is like mine it is almost a sure bet that the moment you subject that tank to the stresses of moving it, that tank is almost certain to fail immediately.

If you are critical for space a better solution would be to build a small 4'x4' utility room on the outside of the structure and place the tank in there.

Oh yes. there is one more small point I hate to mention because no one will ever admit to it but:

In mid summer the attic temps can commonly reach 130 to 150degF. That is fantastic for WH economy because the ambient temp is higher than the desired temp so you have zero standby loss, but many service techs, myself included will pad in another 10 or 20% labor fee for having to work in that environment.