Quote:
I'm guessing that his panel has a similar arrangement internally,
with both sides dedicated for the mains breaker, even if the existing
100A device is only using half of the space currently (pun intended).



Mmm.. on reflection, I'm not totally convinced of this, though.

Actually, it's probably more a code/safety issue that has those
two extra slots labeled as "do not use". The mains breakers are
supposed to be isolated from the rest, and perhaps leaving the
two adjacent slots blank is a way to (barely) satisfy code there.

New panel time.