I. KNOW. HE. ALSO. OWNS. THE. PROPERTY.
I. SAID. THAT. IN. MY. POST.
Unless the OP can’t read, she will have read the numerous posts stating that she can’t deny her ex access to the property as he also owns it. That is the legal position. He has a legal right to enter the property because he also owns it.
He doesn’t live in the house and the OP hasn’t said if he had a genuine reason (other than being a dick) to enter the property.
We all have legal rights and have remedies available to us if those rights are infringed. Whether someone pursues those rights and what the remedies actually are can be different in real life to what is “on paper”. How many posts are on MN about crappy neighbours who effectively break the law and posters tell them “ignore it for an easy life” ie let them break the rules without seeking the legal remedy you are entitled to?
The OP knows her ex and how he’s likely to react/what he’s likely to do if she changes the locks.
OP, as I’m sure your well aware, your DP has a legal right to enter the property.
You've got a lost key, that’s not secure.
I know what I’d be dealing with first.