HOUSTON -- Major League Baseball is working on a scheduling reconfiguration for the 2013 season and beyond that likely will eliminate the Mets and Yankees as well as other "natural rivals" playing home-and-home, six-game series annually, baseball sources told ESPNNewYork.com.

With the Houston Astros moving to the American League West next season and the leagues becoming balanced at 15 teams apiece, natural rivals throughout baseball no longer will be guaranteed six games a season and home-and-home series, the sources said.

That goes for obvious intracity rivals such as Mets-Yankees and Cubs-White Sox, as well as for more-forced natural rivals such as the San Diego Padres and Seattle Mariners.

The Mets and Yankees will continue to play six games a season -- three apiece at Citi Field and Yankee Stadium -- when the AL East and NL East line up for long-form interleague play every three years.

But in the other seasons, a major league source added, the competition likely will be limited to three games at one ballpark, or two games apiece at each ballpark.

Sources cautioned that the 2013 Major League Baseball schedule, and the precise new configurations, are still being discussed.