Tried two other approaches:
Not working: "Renegotiation" of an existing WebRTCEndpoint (new offer/answer cycle) -> throws a "renegotiation not supported" exception in Kurento.
Working: Creating a completely new PeerConnection on client side -> it can connect and exchange media with the new WebRTC endpoint in Kurento (after having released the 1st one).
Thats fine for me. But please let me know if you recommend another approach.
Sebastian