I'm afraid this won't work.

They'll happily reissue the ticket for you, but there will be a $150 change fee.
This is simply how revenue management prices things (I suppose this is called dynamic pricing). Sorry you'll be stuck with the difference - but it's why those of us who plan trips usually stalk the prices until they hit our sweet fare spot.

As a slight aside, I lucked out with this yesterday. A trip I was looking to book hit my threshold, I booked it, and then for some reason re-ran the fare and it was $80 lower. I booked this and cancelled the previous reservation, but there was only about 3 hours between ticket issuance.