I'm reasonable sure they take that into account and would go to extra lengths (or offer sweeteners beforehand to change dates or flights for paid Envoy pax involved) to avoid that situation.

Currently, your Saturday 11/19 PHL-DUB is showing C9 (wide open, with at least 9 Envoy seats for sale) with a 767 aircraft (18 Envoy seats) listed. Say conservatively they've sold 9 Envoy seats (18 total minus 9 for sale) already on US722 11/19 ... that would still enable them to switch to a 757 with 12 Envoy seats and take care of all paid Envoy pax so far.