As mentioned, the agreement between the Expos and Edmonton ends at the end of the 2004 season, so the new AAA RRXpress could be Houston, and they will certainly be shooting for that. That being said, the agreement between the Astros and the current Express is also up at the end of 2004, so it's conceivable that the Ryans make a package deal with somebody else. It looks like Houston digs on Round Rock, so I would expect the former.
BTW, here's what the Express said in their release about the AAA team:
"Another major question fans have is who will be the Triple-A Express' parent club? The Trappers are currently under contract with the Montreal Expos through the 2004 season. After the club comes to Round Rock, the Express will be free to sign a contract with any franchise. The Express is dedicated to signing with a club that has regional appeal and that will continue to provide Round Rock with quality players."