Okay, here are some of my modifications, if anyone is interested. Please keep in mind that I'm still working on these.
Drill 1 (line taps, etc.)- I do it as-is, since it's fairly low impact. If you have trouble with the squat position, you could try side lunges instead (one knee bent, one leg out to the side)
Drill 2 (lateral ski hops, etc) - the lateral skaters seem fine. To add intensity, I hold some light dumbbells (3 or 5 lbs) and accentuate the arm movements. When she turns them into lateral hops, I'm still working on an alternative, but now I keep to the lateral skaters and really get down low. I'm working on the part where she jumps on and off the step but haven't got the count down yet. I step onto the step and go immediately into a couple of side leg lifts.
Drill 3 (using the ball) - I'm still playing around, but I'm basing the footwork on knee repeaters, a few to one side, then a few to the other - really reaching back, and getting down low. I'm using light dumbbells but trying to keep the arm work the same. I've had to make a few alterations if the heavier weight doesn't feel right.
Drill 4 (L-step combo with plyo jacks, etc.) This one I'm able to keep mostly the same, but just avoid jumps if they bother me. If any sort of "Jack" bothers you, you could try fitting side leg lifts into the pattern... As for those fast feet drills, well, good luck. The last time, I replaced them with those kick-taps from LIC, sort of like a kickboxing move, and just kept going until she was done.
Drill 5 (forward jumping and bounding) - for the forward jumping, I stay in place, facing the TV. I squat all the way down so my fingers can touch the floor and then push up as fast as I can go. I keep my toes on the floor, but raise up high, so there's not actually any jumping involved. This might be difficult for those with knee problems. For the forward bounding, I do side kicks and tap the ground (another kickboxing move), alternating sides according to Cathe's count. She goes forward, then back the other way, and then I switch sides.
Drill 6 (football drills) - I do them because I love them and then pay for it later. But I do something low-impact, like those LIC tap-kicks, when she does the high-knee jog. The segment where she does those terminator things is a bit easier to modify, but still keep to her same basic moves.
I hope this helps - if you have any questions, let me know.