Let's say that it is a 'new protocol heavily based on L2TPv2'. It shares a lot of the messages, constructs, flows with L2TPv2 but it is not strictly backwards compatible. It does have an L2TP 'compatibility' scheme as indicated in section 4.7 of the RFC, but that comes with limitations.
The most significant changes are actually listed directly in the RFC:
Notable differences between L2TPv2 and L2TPv3 include the following:
Separation of all PPP-related AVPs, references, etc., including a portion of the L2TP data header that was specific to the needs of PPP. The PPP-specific constructs are described in a companion document.
Transition from a 16-bit Session ID and Tunnel ID to a 32-bit Session ID and Control Connection ID, respectively.
Extension of the Tunnel Authentication mechanism to cover the entire control message rather than just a portion of certain messages.
Your second question is unclear, network changes is a very generic term. Very high-level:
- L2TPv3 over IP/UDP can obviously handle any network change between L2TP endpoints, as long as there is IP connectivity.
- L2TPv3 can handle any L2 change at both endpoints, as long as the endpoints themselves don't change.
- L2TPv3 can not dynamically change the endpoint for an ongoing session, so suppose your L2 changes in such a way that L2 traffic ends up at a new endpoint, that would require a new L2TP tunnel as far as I know.