Houston was in need of a closer after losing Mark Melancon to the Red Sox in a trade.
Myers does have experience as a closer, recording 21 saves with the Philadelphia Phillies in 2007.
In two years with the Astros, Myers has a record of 21-22 with a 3.79 earned run average in 67 games, all but one of which have been starts.
Houston asked Myers shortly after the start of spring camp if he would be willing to make the switch. He agreed to the move Monday.
"There's no way we would want to do something like that if he wasn't completely on board," Mills said.
In 307 career games (249 starts) with the Phillies and Astros, Myers is 94-85 with a 4.24 ERA.