We also closed our Caja Murcia account but moved to Santander in Camposol.
From our experience Polaris aren't in the slightest bit interested you have changed bank details. They will ask you to contact the electric and water company yourself. (We had filled in the initial forms with our Solicitor for all the utility connections as well.) All they could give us were phone numbers but no addresses to write to.
We ended up driving to Alhama and went to the Water Company's offices to notify them we had moved banks.They don't speak any english in the office so there is no point phoning unless your Spanish is better than mine and they never responded to EMails.(even when written in Spanish). With the help of a Spanish Dictionary which we passed back and forth between me and the lady in the office we managed to get it sorted.
For the electric company (Iberola) we went into the town hall (again in Alhama)and a very helpful man phoned them for us and gave them our new bank details. The town hall man only spoke spanish but we managed to communicate with him by using google translator on his computer.
The issue you may have with both companies is they may not be able to trace you until you have been connected.
Best of luck.
Jon
Best of luck