If you really wanted a Business Select fare you could cancel your check-in and change your flight, booking the Business Select fare instead. What I think you're actually trying to do is to upgrade your boarding position which uses the same A1-15 boarding positions as Business Select, but is not changing your fare type to Business Select. To purchase the upgraded boarding position you will need to inquire about available boarding positions at the gate before your flight.
--TheMiddleSeat
