In principle, as you seem to realise, it's a reroute - US$125 change fee, plus whatever BA charges you these days for actually doing any work, plus a whack of UK premium class tax. But only if the gap between scheduled arrival and departure is more than 24 hours (doesn't matter if it's next calendar day if it's less than 24 hours). Whether they would notice - who knows - it's a gamble, and if they do and force you to go through the reissue you'd better have a couple of hours at least to spare when you check in, otherwise you're going to miss the onward flight...

Other Asian cities that have F service (at least some days): KIX, SIN, BKK, MNL, CGK.