For both sidebands to be antiresonant in the arms, the first modulation frequency has to be:

f1 = (n + 1/2) c / (2*L)

where L is the arm length and c the speed of light.  For L=38m, we pick to cases: n=3,  then f1a = 13.806231 MHz;  n=2, then f1b = 9.861594 MHz.

If we go for f1a, then the mode cleaner half length has to change to 10.857m.  If we go for f1b, the MC length goes to 15.200m. A 2 meter change from the current length either way.

And the mode cleaner would only be the first of a long list of things that would have to change. Then it would be the turn of the recycling cavities.

Kind of a big deal.