Did not find the direct method ... will wait for experts to answer.
What if you have cutting tools of different length and divide the toolpath based on the holder? Just a suggestion.
You are asking to divide a path by cut levels. There is no command for this, but you can probably do it with time or distance, assuming there are non-cutting moves between the levels.
For a starting point, look at the length of the path in the Operation Navigator column, and the number of levels, and estimate the length of the smallest level. Do Tool Path > Divide, select a length about that of the smallest level, and a very large tolerance. The system tries to divide the path at non-cutting moves if it finds them within the tolerance, so by setting a large tolerance, the system will divide the path between the levels.
For example, a cavity milling operation is 5000 mm long and has 5 cut levels about the same size. Divide the tool path, set the length to 1000, and the tolerance to 80%. You may have to adjust a little, but I think you can what you want.
If this does not give you what you want, the API has access to the cut level markers, so you could write a program to edit the tool path.
Haha, I've definitely given it a thought. I just know Siemens is working on their own version of adaptive clearing so I'm holding out until then. I've been fortunate to test what they have released so far and I'm impressed with what they have done in a short time. Besides a few hiccups here and there, I like the path they have been able to generate.
I won't lose hope for buttom-up in adaptive milling..
Anyway, back to the question opened the discussion - buttom up is more than rearanging existing cut levels. With top-down you have the same coverage with every level. With bottom up the 1st level (at the bottom) already cuts some / all the material avove. Therefore maintaining the entire upper level(s) would result with a lot of air cutting and significatly reduce the attractiveness of bottom up.
As Mark said - the way to acomplish such would be by NX Open trigerring the level markers, But I wouldn't go there.