Sonny,
Having completed one cycle of KB Muscle, here are some of my observations:
Moving up a bell size for KBM can be a big step. The bell size test is 10 solid double MPs. Can you currently pass this test with the next size bells? If not, you might want to focus on this as a goal in your training for the interim between your next KBM cycle. An Easy Strength style program would be good for this, or the Fighter Pullup program done with double MP (with your target bell size or one size up) and starting with your current "comfortable" rep max.
Another option for adapting to the next bell size is to expand KM by 6 weeks, and do the following (this is from one of Geoff's emails addressing this issue):
"Keep the sets of KM the same for the
first 5-6 weeks.
Use the following rep scheme:
Week 1: 3 reps
Week 2: 4 reps
Week 3: 5 reps
Week 4: 4 reps
Week 5: 5 reps
Week 6: Off
Week 7: Break into true KM."
Geoff's complex/chain programs can be effective for muscle building or fat loss, depending on how you eat. So doing another one, with a more restrictive diet strikes me as basically more of the same.
You could still probably get away with it -- I do think these kinds of programs are sustainable if you are smart about rotating the specific programs, and Geoff is very smart about waving the workload within each program. But if you specifically want to hit KBM again with heavier bells, I'd do a change of pace strength-oriented program, with a focus on meeting the pressing standard for KBM with your target size bells.
Finally, I think you can return to KBM in less than 12 weeks, especially if you are not necessarily focused on packing on the maximum amount of muscle possible, and if you have a little bit of a "strength cushion" with the bell size you are using.