Change:
all versions, and one "compatible" that allows saving the round trip but only applies when the versions are compatible. <del> The client initiates a QUIC connection by sending a first flight of QUIC packets with a long header to the server INV.  We'll refer to the version of those packets as the "original version".  The client's first flight includes Version Information (see vers-info) which will be used to optionally enable compatible version negotation (see compat-vn), and to prevent version downgrade attacks (see downgrade). </del> <ins> The client initiates a QUIC connection by choosing an initial version and sending a first flight of QUIC packets with a long header to the server INV.  The client's first flight includes Version Information (see vers-info) which will be used to optionally enable compatible version negotation (see compat-vn), and to prevent version downgrade attacks (see downgrade).  We'll refer to the version of the very first packets the client sends as the "original version" and the version of the first packets the client sends in a given QUIC connection as the "client's chosen version". </ins> Upon receiving this first flight, the server verifies whether it knows how to parse first flights from the original version.  If it does not, then it starts incompatible version negotiation, see <del> incompat-vn.  If the server can parse the first flight, it can either establish the connection using the original version, or it MAY attempt compatible version negotiation, see compat-vn. </del> <ins> incompat-vn, which causes the client to initiate a new connection with a different version.  For instance, if the client initiates a connection with version A and the server starts incompatible version negotiation and the client then initiates a new connection with version B, we say that the first connection's client chosen version is A, the second connection's client chosen version is B, and the original version for the entire sequence is A. If the server can parse the first flight, it can either establish the connection using the client's chosen version, or it MAY select any other compatible version, as described in compat-vn. </ins> Note that it is possible for a server to have the ability to parse the first flight of a given version without fully supporting it, in

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.