I’d echo earlier comments that it is unlikely to be cost effective. I have in the past - as have others - lucked out trying this on JFK-LHR. If it suits the airline and they’d rather get you on an earlier/later flight when yours is oversold they may move you. But this is certainly not policy and can’t be relied on. By the book, you’d have to pay whatever change fee plus fare difference, which on the day is likely to be massive. An avios ticket may make this impossible rather than merely expensive.