This is from the alpa pdf on 117 rules giving the example that appears to be in dispute. Looks to me like if they reassign you prior to your trip ending, they can add what they want if it's within your FDP limits. But you can't block in, hit the train home, then get a call to go fly, without having an adequate rest period.

That said, after reading every single line of the 117 rules, this scenario is not exactly clear and specifically addressed, thus the dispute and conflicting guidance. The conference call answer JC I think gave doesn't jive with alpas publication.

Q-90. A flight crew member on reserve is assigned a 3-day trip. On day 3, the flightcrew member flies one segment and returns to base. After block-in, the certificate holder assigns 3 additional segments which can be completed within the FDP, flight time and cumulative limits. Is this permissible?
A-90. If the flightcrew member has completed the FDP (blocked in), he/she cannot be placed on a RAP to allow further flight assignments until he/she has received a 10 consecutive- hour rest. However, before the end of the FDP, the certificate holder could reschedule and add the 3 flight segments. In that event, the flightcrew member would have to recalculate the FDP, flight time and cumulative limits to determine if the additional flight segments can be legally flown. The flightcrew member would also have to reaffirm his or her fitness for duty before beginning each flight segment.