Midfielder Michael O'Connor signed for the club in July 2009.
The Crewe man signed for a fee which could rise up to £225,000 and inked a three year deal to join the Iron.
The Northern Ireland international made more than 80 appearances for the Railwaymen, scoring three goals.
One of those strikes was against United last season when they were beaten 3-2 at Gresty Road.
He also had a loan spell at Lincoln towards the later stages of the last campaign, in which he scored once in 10 appearances.