Can a landlord increase the fee for late rent after a lease agreement has been signed?

Everything I've seen says no.

The late fee must be in the lease, must indicate at what point it will be assessed, and how much the fee will be.

I would just make sure I pay on time, then what difference does it make?

If the late fee specificly stated in the lease, they should be, I have seen a few leases that had a vague term of fee to be determined, but I don't think that would hold up in court. But would end up there if everyone does not agree

No terms in the lease can be changed until the lease renews.