When we complete our solicitor advised us to get the locks changed immediately. He told us not to furnish at all until all the snagging was completed and that PW do have master keys.
We had our locks changed by a company who part furnished our apartment who are no longer trading.
I watched the guy do it and it is very simple, all you need is an allen key (to remove the handle on the inside of the door) and a cross point screw driver to remove the barrel,(one screw). Whole job about 10 minutes.
The biggest problem is finding a replacement barrel. I believe our company got one from San Javier and cost about €35 , but I have found a small hardware store in Puerto de Mazarron who I think says he can get them. Sorry for being vague but he speaks no english and my Spanish is not that good. I would suggest either take the barrel or some pictures.
The shop is by the indoor market, by the bus station on the corner opposite.
Hope this helps.