If you switch flight in the CI system and get a BN for the earlier flight you will likely not face a problem. BUT, your reservation has not changed and your status for the earlier flight is only request (even if you can not see that in the OLCI interface). So in the case of overbooking on the earlier flight they might bump you (you will be behind even S-fares with no status). So I would recommend to call and change in the reservation. Remember to undo CI first and then do a new CI when the updates in the reservation has been transferred to the CI system, typically takes 1-5 minutes.
If you do not undo CI before changing the reservation you might end up with a situation where youy will not be confirmed on any flight but being put on request no matter what you do or no matter whom you call.