Normally if I'm looking to switch a player, I try to keep him within the same position group. Dline, LBs, DB, Oline, etc. Seems to work out smoother for me.
Here's a chart of how different positions rely on different rating weights.
Ratings
According to the chart, Speed/Zone Covg/Man Covg only make up a total of 16% of a MLB's OVR rating.