Change travel of '04 fox from 100mm to 120mm?

Yes!

Oh wait, what model?

If it's a Fox Float, then yes. If it is an F100, then no.

You can squeeze 110mm out of an F100, but in my experience the bushing play in combination with a 7" front disc made it really unstable when braking.

Float.

In that case you should be able to remove some spacers and change the travel.

1. Deflate the left leg.
2. Remove topcap, unbolt the footnut and pull the air spring out the top. I usually save the oil from the air chamber and re-use it.
3. There should be a black plastic disc with some spacers under it. Pull out a 20mm spacer for 120mm.
4. Reinstall the air spring, put 20ml of oil in the lower and a couple CCs of heavy weight oil in the top.
5. Inflate. Your pressure settings may change.

The answer is: Maybe.

Some Float 100 forks are 120mm travel models which have been limited down, but most Float 100 forks have a max travel of 100mm. Like Berkley said, pull the air spring assembly out and see if you have a spacer on it. While the air spring is out, extend the fork to make sure your damper cartridge has the necessary travel as well. If everything is good, then reassemble it, using 5cc of 80w gear oil on top of the air spring.

If your air spring doesn't have the spacer, then you're probably out of luck, but if your damper cartridge has the travel needed, you can always order a new air spring assembly direct from Fox for about $50.