When you are in franchise mode there is a glitch/secret that I guess the Devs. forgot, you can change the position of players. Just go to rosters, View Roster, and put up the Player Managment screen for a player on another team (e.g your the Bears, Go to Peyton Manning's screen) Go to edit Player, and Go to Player Information. Here you can change the name and such, also, manning's position, change him to a WR, or whaever you choose. His rating will significantly decrease and you can trade for him using a very low player (e.g. Jonathon quin, or a 7th rounder)
If the player's Position can not be changed, this is because the team needs to have a person to fill in for him, or an extra person in the needs column, you can give a team an extra person by trading...or you can switch someone's position into the needed position.