Actually, that's the other way around :-> SimConnect clients built with the SP2 version of the FSX SDK will also work with ESP v1 (it should actually support all previous versions, but I've only tried it with SP2 SDK built clients).
SimConnect clients built with the ESP SDK will only talk to ESP (FSX isn't foreward compatible with versions that weren't released yet when it was :-> ). I suppose FS11 and/or TS2 might have recognized ESP v1 clients, but that's a moot point now :-<
Adding to what Tim stated, it seems you can always connect to get the version number of the server. Any attempts to use other functionality will give you a server version error in my tests, but you can always get the version number. This allows you to load a different client dll as appropriate for the server if you do dynamic client dll loading (ie, not using the built-in manifests).