Change:
2.3. When the server can parse the client's first flight using the <del> original version, it can extract the client's Version Information structure (see vers-info).  This contains the list of versions that the client knows its first flight is compatible with. If the server supports one of the client's compatible versions, and the server also knows that the original version is compatible with this version, and the client's first flight is compatible with this version, then the server converts the client's first flight to that version and replies to the client as if it had received the converted first flight.  Note that this conversion process cannot fail by definition of the first flight being compatible.  The version used by the server in its reply is refered to as the "negotiated version". The server MUST NOT reply with a version that is not present in the client's compatible versions, unless it is the original version. Clients will be made aware of compatible version negotiation by seeing a change in the QUIC long header Version field.  It is possible for the server to initially send packets with the original version before switching to the negotiated version (for example, this can happen when the client's Version Information structured spans multiple packets; in that case the server might acknowledge the first packet in the original version and later switch to a different </del> <ins> client's chosen version, it can extract the client's Version Information structure (see vers-info).  This contains the list of versions that the client knows its first flight is compatible with. In order to perform compatible version negotiation, the server MUST select one of these versions that (1) it supports and (2) it knows the client's chosen version to be compatible with.  Once the server has selected a version, termed the "negotiated version", it then attempts to convert the client's first flight into that version, and replies to the client as if it had received the converted first flight. If those formats are identical, as in cases where the negotiated version is the same as the client's chosen version, then this will be the identity transform.  If the first flight is correctly formatted, then this conversion process cannot fail by definition of the first flight being compatible; if the server is unable to convert the first flight, it MUST abort the handshake. Clients can determine the server's negotiated version by examining the QUIC long header Version field.  It is possible for the server to initially send packets with the client's chosen version before switching to the negotiated version (for example, this can happen when the client's Version Information structured spans multiple packets; in that case the server might acknowledge the first packet in the client's chosen version and later switch to a different </ins> negotiated version). Note that, after the first flight is converted to the negotiated

Discussion:
Original version was defined as the version a client attempts prior to a VN packet. If there is a VN packet, I think that it is still possible to do a compatible upgrade from the version in the client's second attempt. In that case, "original version" isn't the right term to use when talking about compatible upgrades.
We can sidestep this particular issue by removing the "unless..." clause. That clause isn't helping because the client's chosen version MUST already be present in the compatible versions list. However, this issue raises a good point: it's possible for incompatible VN to happen and then be followed by compatible VN. We should clarify that this is allowed, and clearly define what the "original version" is in that scenario.
Consider the case where the client and server both support v1 and v2 and prefer v2. In the current draft, if the client puts v1 in the long header and the server responds with v2, that's called "compatible version negotiation". OTOH, if the client puts v2 in the long header and the server responds with v2, that's called, well, I'm not quite sure what. I guess not negotiation? I think a better way to think about this is that the client is sending an Initial that is compatible with both versions and so both of these cases are compatible version negotiation.
In the scenario where the client starts with v2 and the server responds with v2, I think of that as just "creating a v2 connection" without compatible version negotiation involved. Let's discuss this with the WG at IETF 113.