When I feel a 6” step is no longer as challenging for a certain workout, I try using 8” for as long as possible in the workout, moving back down to 6” like you did when it becomes too much. It’s a very gradual process. Sometimes I can only make it through the warm-up at 6", but eventually I can do the warm-up and the first combo, and then more and more each time I do the workout. (This is how I originally transitioned from 4 to 6”).
Another thing I’ve done is to use 8” for either the intervals/blasts OR the step combos in IMAX2, Step Blast, Low Max, BM2 circuits, etc. (switching to 6” for the other portions of the workout).
These are a few of the challenges I've encountered when moving to an 8” step:
1) My body tends to get very used to certain repetitive step movements (is this a muscle memory thing?). So I have to be careful when switching to a higher/lower step height than I’m used to because I’ll automatically step up (or down) in anticipation of a higher/lower step height than what I actually have set up. Anticipating a higher/lower step height can be rather jarring, like when you step off the last stair in a stair case thinking there’s still one more stair left.
2) I like jumping/lifting off the step for a lot of movements, especially in workouts like Step Blast and BM2. However, moving to 8” often requires initially taking out most of the impact, which can make the workouts seem more tedious than usual. For instance, jump ropes around the world in BM2 are a lot more fun when you get to actually jump vs. modifying into something like plain old knees around the world, which BM2 already has plenty enough of (no offense to BM2, which is actually one of my favorites).
3) For me, having to drop down to 6” mid-workout can make me feel like I have somehow “failed” or “wussed out.” I realize this is totally a mental thing, but I can’t seem to shake the feeling that I’m somehow “giving up” by lowering step height mid-workout
Also, I personally find an 8” step unnecessary for certain workouts (e.g. Intensity, IMAX3), which remain plenty challenging for me at 6”. And, if I’m having a low energy day, I just stick to 6” for whatever workout I’m doing and don’t try to push it.