I'm noticing that AA offers refundable fares for flights that I want to book on BA which does not offer a (reasonably priced) refundable option.

I am choosing Premium Economy JFK to LHR and Economy return to JFK. Onward is the PE refundable fare and return is Main refundable fare, which add about $300 extra in total.

Now I'm reading the fare rules - JFK to LHR fare code is OHN4C1P9 and LHR to JFK is OLN0C2M9. Both fare rules say "BEFORE DEPARTURE REFUND IS PERMITTED. IF PASSENGER FAILS TO CANCEL BEFORE ORIGINALLY SCHEDULED FLIGHT THE TICKET IS NON-REFUNDABLE. AFTER DEPARTURE THE TICKET IS NON-REFUNDABLE."

However once ticketed, the endorsement says the following:

Endorsements/Restrictions

NONREF AFTER DEPARTURE/RESTRICTIONS APPLY

Anyone familiar with the refundable fare - what if someone flies the first segment and then cancels the rest, does the ticket become non-refundable after flying the first segment? Or will the price be recalculated and the rest refunded after taking the first segment out?