It depends on how many people your current ADRs are for. If you are going from a group of 3 to 4, that will be no problem. The table size will be the same for either group. If you are going from 4 to 5 that will be a problem, because you will need a larger table.
You really should try to change your ADRs ahead of time. If that fails, you can show up with more people than your reservation is for, but be aware that you may have to wait an extra long time to get seated. Or they may not be able to accomodate your extra person. Disney will do their best, but at restaurants such a Le Ceiller they would have a heck of a time accomodationg extra guests.