I'm having a problem while in on-line check-in. I already have selected my seats in my reservation, but when I try to check-in I get:
"Sorry, we cannot continue to check you in online for this flight as we are unable to offer you a choice of seat at present."

... Which, in my experience with other airlines, smells like either overbooked or equipment change. I have very little experience with BA so maybe this is totally normal, but it's a long trip (HND) and I really have to be there. Should I get worried?