I hope by "altered it" you really mean that you defined your own procedure with a different name with similar code? If not, then I highly recommend that you not actually change the definition of dbo.sa_conn_activity() because if you do you will lose your definition (and it will be revert back to the original) if you ever unload/reload your database or do a ALTER DATABASE PROCEDURE ON.

or an "ALTER DATABASE UPGRADE PROCEDURE ON" - yes, one of my current favourites:)

I had to change the actual procedure. Yeah, I know. I tried to define my own but there's a "next_connection" function call inside and I cannot access it in my own procs (is that possible?). Actually this is all about monitoring incoming queries to the server. My .NET program saves a copy of the original definition and pushes its own version. So both versions are safe. There might be a better way to do the same thing, but I haven't come across any yet.

Of course you can use the next_connection() function in your own code - typically in a loop, e.g. something like

declare nConnId integer;
set nConnId = (select next_connection(null));
while nConnId is not null loop
  -- do something with nConnId
  set nConnId = (select next_connection(nConnId));
end loop;