The Austin American Statesman reported that the Astros may change Triple-A affiliations, perhaps as early as September. Citing anonymous sources, reporter Kevin Lyttle's article says that the Rangers' interest in buying the Oklahoma City AAA team could allow the changing of affiliations. (Article linked here is behind a pay wall.)
The driving force, according the article, is the Ryan family's desire to keep their business interests with the same major league club, and the Rangers' interest in owning their Triple-A franchise, in particular the Oklahoma City franchise which is up for sale. A Pacific Coast League official is quoted as saying "that's spot on."
The Rangers have a player development contract through 2018 with Round Rock, leading to the conclusion that the Rangers hold the cards for this to happen. The Rangers could terminate the deal early if all parties consent. John Daniel, the Rangers' GM, seemingly throws some cold water on the speculation, pointing out that the Austin area is a very desirable sports market, and asks why the Rangers would depart from that market after spending so much effort to get a toehold in it.
The Ryan family has an ownership stake in the Round Rock Express. Reid Ryan is president of the Astros, and Nolan Ryan left the Rangers, joining Reid as an advisor to the Astros. Reese Ryan, his youngest son, is the CEO of the Express.
The article notes that Astros' owner Jim Crane wants to own a Triple-A franchise in the Woodlands, but claims that Reid Ryan has persuaded him that returning to central Texas would be less difficult than the obstacles for moving a franchise to the Woodlands.
I admit to having some skepticism regarding the American Statesman article. We don't know how definitive the sources might be, and the quotes from the Ryan family and Daniel make the premise seem iffy. That said, it might be understandable if the Rangers want to own their own Triple-A team---for much the same reason that Crane wanted the Astros to own their own Triple-A team. As an Austin resident, I would certainly welcome a return of the Astros' Triple-A team to Round Rock.