I was just wondering in a lease,if u have a set date for a late rent charge,can a landlord change the date without a new lease?

No.

At least not legally.

The problem is, how bad of a relationship do you want to have with your landlord?

If either of you gets to the point of going to court over a late fee, the remainder of your time in that residence will be miserable at best.

How drastic is the proposed change?

I would say probably not, but the definite answer would depend on the wording in your lease as well as your state's landlord / tenant laws (see sticky at the top of this forum for a link.) It's possible that with enough notice your landlord can change some "rules" in the lease. Usually nothing can be changed that affects dollar amounts or dates.

Have you consistently been paying your rent late? If your payday falls later in the month, or you're having some kind of difficulty I'd try to work things out with your landlord, rather than getting into a battle with them over late charges and when they start.