Changing the locks would entitle them to bring a claim against you for compensation for unlawful eviction.
You can only ever change the locks if the tenants have actually vacated, and even then it is sometimes wiser to get an order for possession.
If tenants fail to vacate after giving notice to quit then to get possession you will I am afraid have to get a possession order. You can do this on the basis of the notice to quit that they have served.
玩点文字游戏是解决不了难缠的房客问题的,搞不好受害者成了加害者。