Can the locks be changed before the date and time on the writ of ejectment?

Only if you give the tenants a key. Until they are legally removed you can't lock them out.

Or if the tenants have already moved out and no longer live there.

The judge ordered to be out by Thursday 10 am if not he would serve a writ of ejectment. He did this on Friday for Monday 10 am but the locks were changed sometime Friday. The ex girlfriend still lives there but would not allow the party to access the property

First, to add follow-up post an answer to this thread, Don't ask a new question. I've merged your threads for you.

So the tenant changed the locks? That shouldn't matter. You hire a sheriff to serve the write and physically remove them. Then change the locks.