Part of this depends on when and why the pairing was changed. The test, in the contract is if the change is "beyond the control of the company". Of course if your trip was a * trip and its premonth then all bets are off. They can do what they want. However, if you have a rotation that is say mid month and the company arbitraily changes it (IE, marketing wants to put a bigger jet on it) then you are entitled to reroute pay.

A lot of times you will see the company put 23K on your schedule. I think that schedulers do that sometimes in error. Marketing changes are not 23K. The recovery rules and pay adjustments are different for the reasons your schedule changed. In the case of them extending due to marketing changes, for example (different scaled jet, adjustment of frequency etc) and that change is within the control of the company, then you are entitled to reroute pay.

I would call ALPA, ask for the scheduling guys and have them look at your trip. My guess is that if they changed it and added another day this far in advance, its probably marketing making the call. In that case, ka ching.

Hold their feet to the fire. They don't like to pay pilots extra if they can help it. I have had this happen before and have had to fight a bit to get the pay. If you are successful, make sure you notify the other pilots as well so they can get paid.

Good luck