Can I switch from monthly to a credit pack partway through?

Yes, and what decides is the shape of the class: a continuing class offers both monthly and credit packs, a fixed-session class always uses credit the size of its session count. Changing the shape of the class changes the options offered the next time a pack is sold.

A pack already running does not change with it — it finishes under its own rules. So the switch happens on the NEXT pack, and no student suddenly loses sessions they already paid for.