The key to going from parrot stepping up strictly for treats to stepping up just because is variable ratio reinforcement, conditioned reinforcement, and habit. I should probably write an article about this but this is really more the why than the how. If you follow the steps I outline, you should eventually achieve this. Recap of the how:
1) Targeting on perch
2) Targeting on hand
3) Step up onto hand first to do trick (even target) for treat
4) Step up for various opportunities (sometimes trick, sometimes treat, sometimes go places)
5) Step up out of habit
Ideally you should never be giving treats specifically for stepping up. Ideally you give treats for doing "good things" like tricks and treat step up as a means of performing them. This makes the bird learn to step up for other good stuff to happen rather than exclusively for the lure. Since step up is very easy, the bird needs little reward to maintain it in the long run so eventually you dilute how often a specific reward is given. Come up with alternative rewards (such as a head scratch but ONLY if the bird actually enjoys it or to go in cage for meal) for stepping up. And I'd say after 3+ months of doing this, the bird will pretty much just step up always just because and not because you are dangling a treat in front of its beak. It will just be used to good things typically happening in your presence and after stepping up.
A VERY VERY VERY important thing is NOT to EVER use step up for something bad. For example, asking the bird to step up to be "punished with a cage time out" is moronic. It's as ridiculous as saying to someone "come here so I could slap you." You may get away with it a few times but eventually it will
teach the bird to fly away (as was the case with Kili) or to bite if it can't. There is no reason for it to want to step onto your hand to be punished. This is why
the cage must always be good or at least neutral.
You cannot believe how many times people tell me their parrot bites when they want it to step up to which I ask, "what do you do with the bird after it steps up" to which they reply "put it away in the cage." People are treating these astronomically intelligent animals like luggage. They need to be given a good reason to step up and not just because you paid money for them. The reason we need to use targeting and treats at first is because they don't know that what we'll do with them next will be worth their while. So we "train" them using targeting and treats so that we could earn their trust and the opportunity to show them how good it can be in other ways. Examples will include scratches, talking to them, taking them interesting places, handing a toy, giving a meal, etc. And this won't even have to happen every time. But as long as it is always good or nothing at all but NEVER BAD, the parrot has little reason to not step up for you. Compared to the boredom of being in the cage it's almost always worth it. Think back to being a kid and enjoying watching other kids play almost as much as doing it yourself. I think it becomes like that for the parrot. But until that can be developed, use training as a means of getting the parrot's attention and trust in the quickest, simplest, most reliable way but always think why the parrot should chose to cooperate with you and it'll work out.