Every instructor develops a style of instructing; some are good, some are ok, a few are poor. As an experienced aerobics group exercise leader, you already have a style. As a popular aerobics instructor at your facility, your style has gotten you compliments and increased attendance in your classes. My recommendation is not to change your style - it's successful and it is yours. Just adapt the movements that make up your style into another format - seated.
To begin developing your seated style, take an aerobics or step class while seated. See what movements and movement combinations are awkward and which are intense. Make certain that the class that you take is taught by an instructor who actually uses upper extremities in the choreography and doesn't just depend on intricate step patterns or footwork to raise the class' heart rate.... you know who you are! While you are taking this class, try to exercise with a restriction on motion to just one side, as if you were a stroke athlete. Vary the intensity by varying your lever length.
Don't assume that disabled athletes will know how to adapt the movements that you demonstrate (either in a studio class or in a seated class) to the most appropriate intensity, speed or location for them. Be the instructor, not just the person at the front of the room; before class, instruct them on modifications and include modified movements in the sequence of your choreography as instructive examples.
Examples of movements and movement patterns to consider incorporating into your seated choreography are:
Yes, the basic staple of Low-Impact Aerobics translates well into a seated format. It's more of a Marching Band March than the Parris Island style that we learned back in Basic Training. The Kinesologic Movement is Shoulder Flexion/Extension. Elbows are bent and the arms move in opposition. Almost all of the disabled athletes in a seated class can master this sequence. It is an effective warm-up, cool-down and place-keeper in between segments. For those seated athletes who have some leg movement control, they can lift legs in time with the march. Instructors should be on guard for seated athletes who turn this into a torso twist movement. Increase intensity by straightening out the elbow joint or by lifting the arms to a march in front of the torso. Decrease intensity by decreasing range of motion.
Presses are easily mastered by most seated athletes. These movements can recruit muscles of the shoulder girdle, the shoulder joint and the elbow. The combination of location, intensity variations and speed variations make this exercise very versatile. In fact, in the warm-up period, this is one of those exercise patterns that can be used to preview movement sequences in the aerobic exercise part of the program. Decrease intensity by not pressing above the shoulder level, decrease intensity by pressing in half-time to the music, decrease intensity by not using arm muscles. Increase intensity by doing the exercise to the beat, by using full arm range of motion and by exercising one sequence at or above shoulder level (for those athletes who can do it safely). Increase interest in your choreography by varying the location of the presses (front at the chest, side and down, side and out, etc.). Increase interest by doing one arm, then the other - balance can be maintained if the athletes steadies himself with the non-movement arm.
These shoulder lifts to the front and side are also versatile exercises, but you must take care not to overwork the anterior deltoid, which is the hip flexor of the upper body; it takes over when the other shoulder muscles are fatigued, much as the hip flexor takes over when you have fatigued the quadriceps & hamstring. So, set your choreography up to bracket these movements with arm movements. Vary the location; flexion/extension for eight counts, the abduction adduction for eight counts. Vary the intensity by varying the degree of flexion that you maintain in the elbow joint. Vary the intensity by varying the speed of the exercise, or by double-counting the movement. Cueing on this should always remind the athletes to watch their posture and sit up straight; in fact, one indication of fatigue is athletes slumping in their chairs. Again, one arm at a time is an appropriate variation for most populations, but unless you are confident of the balance and coordination of the group, I would stay away from the one arm flex/extend with the other arm abduct/adduct combination movement, although it is an option for populations that can handle it.
I use these mainly as warm-ups and cool-downs, but they are appropriate exercises for some populations that have limited arm movements. Vary the intensity by varying the speed. Try to work the shoulder through the full range of motion. Vary the interest by rolling or shrugging one shoulder at a time, then both together. Since you should not deviate from a seated, centered position and you are not introducing swinging movements of the arms, moving one shoulder at a time should not cause balance problems.
The advantage of swimming motions is familiarity; not everybody knows intuitively when you spoken command is "PLIE" or "GRAPEVINE" or "U-STEP, RIGHT LEAD", but if your command is "BREAST-STROKE", most of the population knows the movement. Again, be careful of movements (like the backstroke) that lift arms overhead and be careful for the balance with one-arm full-extension movements. Don't forget all the swimming strokes - Crawl, Fly and, of course, Dog-Paddle.
These are exercises that involve movements at the elbow joint. Vary location (side, front, chest high, shoulder-high, shoulder-high front, etc.), vary intensity with the beat, and vary intensity with range of motion. Be careful, again, of asymmetrical movements, since alternating side curls could be a problem with athletes with impaired balance. Don't forget that movement at the elbow is for more muscles than just the Biceps Brachii!
Don't be afraid to incorporate combinations into your choreography. As with combinations in your studio class or in your step class, have these well-rehearsed before you throw them at the class and preview these, either before the class begins or as part of the warm-up. Remember, some disabled athletes with cognitive impairment or with poor muscle coordination may not be able to follow even what you consider simple combinations and sequences. Before class, show them alternate movements and assure them that following your choreography is not a prerequisite to enjoyment of the class - the biggest benefits to your class are participation to the best that you can do.