Can I amend a prior year to "unclaim" the american opportunity credit so that I can claim it this year for a larger credit?

No, once the credit is claimed for that particular year, it cannot be unclaimed in order to claim it on another year.

Maybe.  See: https://ttlc.intuit.com/community/college-education/discussion/can-i-file-an-amended-return-so-to-not-claim-a-previously-claimed-aoc/00/2726042