Coach Tony Sparano confirmed the surprising shake-up Monday as his team began its final week of preparation for Sunday's opener at Buffalo.
Since being taken with the 16th pick in the 2006 draft, Allen has been shuffled between cornerback and safety and consigned mostly to a role on special teams. He started only one game last year and two in 2008.
Smith, a second-round pick last season, started all 16 games as a rookie but made no interceptions. Sparano said the lineup change had more to do with Allen playing well than with Smith struggling.
"This is a good problem. It's not a bad problem," Sparano said. "In my time here, all I've heard about is, 'When is Jason Allen is going to turn the corner?' Maybe we should be happy about this."
Allen, who started 14 games at cornerback at Tennessee, showed steady improvement in training camp, Sparano said.
"He has tackled well, and he has really done a pretty good job in the coverage," the coach said. "The game seems to have slowed down a little bit from that end for Jason."