That what Razzle means when he says bit fields that turn things on/off.

If all they did was change from fixed length strings to null terminated strings, the NetClient changes will be tedious but easy. If they have a lot of places where they pass bitfields that control what data is sent, you are going to need someone to dissassemble the client to figure out what the rules are unless you can collect enough packets to reverse engineer it.

In general, you usually find that they don't just stop sending things or massively change how the client and server talk. The data that was there before is more than likely still there. It is just packed differently.