There's no need to be worried and starting multiple threads in a panic.

From what you're saying, you hold a seat for the 5th, but your e-ticket has not been updated to reflect this change. It's no biggie - just a PITA that you can't use OCLI (for the time being).

Ideally, AA would revalidate your ticket. BA cannot do this as it is an 001 AA issued ticket. BA would have to reissue which is a little bit labourious when it is far easier for AA to revalidate.

However, AA will only be able to revalidate if you rebooked your flight directly with AA. If you rebooked directly with BA (or other OW carrier) then, owing to AA/BA being on different systems, the newly rebooked flight will not appear in AA's system and I can possibly understand why AA would be reluctant to faff around if they cannot see the confirmed space. If this is the case, then BA realy should take ownership.

Chances are that unless the ticket gets sorted out in the meantime, you'll be fine for OLCI tomorrow morning.