As far as I know, you're now stuck--you can't do two challenges so quickly, and once the challenge is over I don't believe you can upgrade it. However, regular purchases of BOS-LAX in F will get you to platinum all on your own, as each round trip will be 7833 EQP, and since you're already in F the lower Gold status won't be a big problem during the time you're reaching for Platinum. The biggest downside I can think of is that you won't be earning the 100% bonus, which is a bummer.

Remember, LAX-BOS is a transcon for AC access purposes, so if your fare is booked into F you would get AC access as part of your ticket (and should be given drink chits as well). Just wanted to be sure you were aware. Of course most tickets that book into first are actually A or P fares, which wouldn't include AC access, but if you happen to be buying undiscounted F, keep that in mind.