When did they change from 24s to 23s and start using a centre runway
They didn't, and they haven't.
Palma's runways are oriented 059°M/239°M, so it would take many years of magnetic drift before they could be designated 05s and 23s. The reference to 23C in post #9 was talking about the temperature.
The northern parallel taxiway
used to be runway 06C/24C, in fact you can still see the rubbed-out piano keys and runway designator on Google Earth.
AFAIK, it hasn't been listed as a runway in the AIP for many years.