I haven't done BC for quite a while, but the moves in it are pretty straight forward, and easy to modify, IMO.
There are several ways to modify higher impact to lower impact while keeping the intensity up.
One way is to emphasize the lower part of the move, so when Cathe and crew are going 'high,' try to go as 'low' as you can. For example: when they do air jacks, you could do side-to-side squats, always keeping knees bent to increase the work in the muscles. Or when they do jump kicks, just take out the jump, but make the squat that leads up to the kick lower.
Another way is to do a lower-impact move, but increase the pace, like doing Tabata squats (go as fast as you can). Or do a squat/floor touch as fast as you can (combining 'going low' with 'going faster').
If you have questions about any specific moves (as I mentioned, I haven't done this one for a while, so I forget all the moves), just ask, and I or someone else can come up with some substitutions for you.
Also, you might find some of the mostly-lower-impact mods I did with Cardio Core Circuit something you could use with this workout as well (and it gives you some other ideas of how to modify, so you could come up with some ideas of your own):
http://thecathenation.com/forum/showthread.php?t=271823
HTH!