@Martin ******
you SHOULD go to the immigration office where you got the extension and get the stamps transferred (as long as you didn't do the eExtension online process)
that's how the rules are written, if you have an extension of stay that is stamped in you move the extension to the new passport at the office that issued the extension before leaving.
You probably COULD show both and get stamped out without too much issue, (they won't deny you exiting the country)