Why the sudden change? Clearly the Lions see some potential in Proctor, but he wasn't going to make the 53-man roster at defensive tackle. He did spend the last two seasons on the practice squad at that position, but he was really buried on the depth chart.
Granted, Proctor is still unlikely to make the 53-man roster and is still buried on the depth chart even after this move. There is a backup spot up for grabs at offensive tackle, but either Corey Robinson or Michael Williams, who moved from tight end to offensive tackle last year, will likely win that job.
I suppose if nothing else, this gives Proctor an opportunity to extend his stay with the Lions. He's got good size at 6-foot-6 and 315 pounds, and perhaps he could once again earn a spot on the practice squad if he keeps the Lions intrigued following his move to the offensive side of the ball.