I booked a trip LAX to LHR a couple weeks of ago for July. Price just dropped $300. I was booked in main cabin. Can I cancel and rebook without penalty with the new change fee rules? Will I then receive a travel voucher for the difference? Thanks in advance for any help!

I figured out how to do this on through my Advantage account, but seems no matter the new itinerary I choose, I get through the whole process of changing, selecting seats, etc and then I get a message that the flight chosen is no longer available...plenty of seats. still listed at same price. Beginning to think there is some false advertising going on? I can't figure it out. Any ideas?