My tenants signed a paper that says the day they will move out and they acknowledge locks will be changed that day. If for some crazy reason they do not move that day can I still legally change the locks since they signed the paper?

Sure if your giving them a key. If not then I wouldn't recommend it. You must wait until after they have gone before changing the locks if you don't want any legal action taken against you for an illegal eviction.

You can not lock a renter out, I will be rude, have you went into the rental business and not even studied the rules and laws. It can at times take months to remove some tennants depending on where and why.

While the tennants may have signed a paper that says you can do something you are not allowed to do, I would get an attorney to write up your legal papers, *** or even use some standard forms from internet or office supply store.
You need to conform your actions and forms to state laws

If they give you notice and then default on the notice you have to evict them like everyone else. They are simply in default of their lease and can't be locked out, eviction takes time and the courts.