Wow okay finally spoke to Bruce in Winnipeg through the rates and reservations department (which is closing end of January and we are losing the most senior agents who know the most about fares rules and changing reservations). He says it is correct that changing an inbound leg on a P fare (after flying the outbound) when P class is available (but not available for sale because of the advance purchase restrictions) only requires a change fee and no increase in fare.

Crazy how many AC agents and concierges I spoke to, they all quoted the need for advance purchase of the P fare and thus not available for our chànge, none would listen when I indicated historical fares could be used and would not require advance purchase. I'll email AC to let them know about the pain I went through and multiple long distance calls with their new inexperienced agents.