I 'modified" MMA Boxing by skipping over the lower-body drills (I preceded the workout with a kettlebell workout, and included the heavy-bag segment as well--minus any jumps-so I still got a good workout). If I decide to include them in the future, I will probably modify many of them (depending on how my body feels). Probably by "stepping" or "1-foot hopping" into moves rather than "jumping" with both feet (for some reason, I find some 2-footed jumping moves more jarring than 1-foot-lead moves.
I did Kickbox(ing) as low-impact as well, and I don't recall it being a problem at all. Just keeping moves low instead of high.
I think that to modify high-impact to low(er)-impact (I'm okay with some moderate impact, or ever 'certain' higher impact moves) and still keep the intensity, you can do one or more of several things:
1) think "low" instead of high. For example, instead of doing plyo squats, squat low and touch the ground. (this will not be an acceptable substitution for those whose knees are bothered by deep squatting). Another option: instead of high jumping, just bring yourself to your toes, but move those arms to get some 'ooomph' into the move, and also create some momentum that your muscles have to work against to decelerate and stop at the top of the move.
2) think "range of motion" and "clean moves". With the same move: squat low to touch the ground, then stand up to your tippy toes with arms reaching overhead. Keep the move clean by imagining you're being filmed and/or you are a model for the movement (or am I the only one who plays mind-games like this with myself?
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3) increase speed of movement. For example, instead of doing 'digs' with a lift, remove the lift, but double-time the moves (so they are more like Tabata squats). Or
4) decrease speed of movement (LOL!) Sometimes doing something slower works better, especially if you can do the move, but not at the speed shown. (I can do some squat jumps, but not as fast as some instructors show).
5) for knee safety, watch for moves with turns (ie: it may seem a slight thing, but I don't like running around the step. The sharp turn necessary to make the move could potentially lead to knee torque, YMMV.) and either make them follow one plane of movement (ie: forward and back or side to side), or keep them in place ('turning" X's become "stationary" Xes). (See my thread on Cardio Core Circuit and how I modify some moves like this), change pivots to mambos, etc.