We are not aware of any requirements under Illinois law as to the number of days for computing a late charge or the exact wording of your loan agreements regarding late charges. If your late charge practices match the language in your loan documents for late charges, and your late charges otherwise comply with all applicable laws, we do not see any problems with this change in wording.
We do recommend that you review your loan documents, since the vendor’s change in wording appears to result in a one-day change in your computation of late charges.