Is it worth it?

I haven't tried it out yet to say how good it is, but I just went ahead and TMed Dragon Tail to Waterfall on my only good Gyarados. I don't like getting rid of legacy moves either, but all I know is that I never used that Gyarados for anything as it was, but now I think I actually will.

You can always just evolve a new one

There may not be a great "raw" difference. But being STAB and weather affected will likely make a huge difference. We will find out soon, when our GP gods start researching the changes to the damage mechanics introducing weather effects.
Looking forward to it, GamePress bosses ;)

For now, I TMed my 100% gyarados and got waterfall. Unfortunately, I still need to get rid of crunch...

Waterfall seems to be worse than Dragon Tail in neutral matchups before factoring in STAB since it has lower EPS. For Gyarados, it looks like Waterfall is a lot better, especially considering the lack of Water DPS Pokemon and the potential for rainy day slaying.

I input Waterfall into the moveset spreadsheet. Says Dragon Tail/Hydro Pump is 91% of Waterfall/Hydro Pump.

Have to wait for PokeBattler for more accurate comparisons, but it is stronger.

As I have a similar question,

I have a perfect IV double dragon move, should I keep it double dragon?