Ah... yes, I remember now. It will be a
lot easier though, rather than running older patch installers overtop of newer ones (which could potentially cause problems), for one of you with the "right" CvGameCoreDLL.dll to just send it to Player 7 via email or something. (It's only a few MB after all.)
Player 7 can then make a backup of his own CvGameCoreDLL.dll and replace it with the one he's downloaded. That way, if (when) he wants to revert his game back to the BTS version, all that he has to do is to replace the downloaded CvGameCoreDLL.dll with the one he's backed up. Far simpler (and safer) than messing around with patch installers.
Remember: Player 7 should be careful to ONLY change the CvGameCoreDLL.dll in the normal Civ4 folder. There are two different CvGameCoreDLL.dll files for both Warlords and BTS - make sure you're in the right folder before replacing them. And ALWAYS make a backup.