I am in a very similar position to you- I was booked on Saturday's (now cancelled) LAX-LHR so rebooked for Thursday (and in the process lost my seat assignment) I just tried OLCI, obviously still too early, and it says it's too early to check-in. Although I will be surprised if this works at 23hours 59 mins because I cannot access my booking in Manage My Bookings- I get a bizarre error message.

All I can suggest is a call to Online Tech Support. Can you access your itenerary in MMB?

P.S. Whatever happens hope you get seats on the upper deck!