I just checked that. The change of the parameters was intentional as we cleaned the API, there were some inconsistencies in the parameter order of functions that we fixed with 3.0.3 release. The bug was that all functions that need a client_id use the client_id as first parameter except of the client_update function, so we decided to fix that for future versions by making the client_id the first parameter in all functions.
I'am sorry for the inconvenience. We try to kep the api stable but this was a bug that we wnated to fix to make the api clearer. The change was described in the 3.0.3 release notes if I remember correctly.
Please adjust your code so that it uses the new order of parameters.
client_update($session_id, $client_id, $reseller_id, $params)