Rent is due on 1st, late fee on 5th. I'm considering changing the due date in the new lease by defining "month" to be from the 10th of one month to the 9th of the following month. Is this adviseable?
Granting tenants an extension of time to make payments does not affect any other provision of the lease.
On the other hand, if you globally redefine what a "month" is, this may affect several provisions of your lease agreement. You should double check the written agreement to make sure nothing else is affected.
This general opinion DOES NOT create an attorney-client relationship. No communication can ever replace the specific advice of a lawyer. Each person's situation is different, and additional facts can change legal outcomes. You should consult privately with a lawyer if you have a question about a legal dispute.