Say I’m a football player named “Jim Jebow.” I’ve made a name as a college QB, but not much success in the pros. Some are suggesting that I should change my position, perhaps to fullback. Is this feasible, and if so, how difficult would it be?
There’s no one answer. It depends on the player and the position. Some have the skill (and physical build) to switch from one to another; most do not.
Could the fellow in my example do it?
Possibly at tight end. That’s what people have been saying since Mr “Jebow” was drafted.
He’d be pretty slow for one of the new fangled hybrid WR/TE guys and he’s pretty small for a blocking TE.
As a fullback? Probably not. He’d be a slower, smaller version of James Casey, who’s actual value is in not fitting into a traditional FB role. Note that even Casey isn’t much of a fullback. But he’s got great hands and reasonable speed in space, so he can add a change-up sometimes. None of that is true for “Jebow”.
Of course, none of this actually matters. The subject in question has steadfastly refused to play any role other than NFL QB. Won’t even consider playing QB in the CFL or arena league.
Of course there are plenty of players who have changed their position. Julian Edelman for example, was a QB in college and is now a WR and punt returner - but has also tried playing cornerback.
Henry Melton of the Bears went from RB to DT. That’s gotta be the most radical change I’ve heard of. A lot of Bears changed positions in their careers actually. Urlacher was a safety in college. Jerry Azumah was a RB who changed to CB and kick returner in the NFL. Of course there’s Hester, who was a CB in college and went to PR/KR/WR in the NFL.
I think you have to break down what basic skills are needed for each position. Here’s my thoughts, they’re incomplete.
QBs need to be able to throw.
Running backs need to be able to run.
Receivers need to be able to run and catch.
Kickers need to be able to kick
Linemen need to be able to block.
Safeties and Linebackers need to run and tackle.
There’s certainly more skills and positions.
I can see a guard and tackle easily trading positions. An experienced QB to FB? How fast can he run, how is he at finding his way through the linesmen, and how is he at being tackled, getting up, and good to go for another down?
And then of course there are players who play multiple positions, such as Kordell Stewart.
The most successful switch at the pro level may have been Todd Christensen, who was a running back in college but made it to the Pro Football Hall of Fame as a tight end.
Fullback sounds like it would be simple enough, but it’s a tough position.
First off, to play fullback, “Jebow” would have to put on another 20-25 pounds and at 6’3" is probably too tall, plus of course he’d have to learn how to block, not to mention that the true fullback is a dying breed in today’s NFL. It could (and most certainly will) make a comeback at some point, but that point is probably years away.
It’s a thankless position; you are part of violent, head-on collisions on every snap, are expected to take on bigger players, and rarely get the ball.
This question has already been answered for “Jim Jebow.” He doesn’t want to play a new position and NFL teams don’t seem interested in bringing him on board as a project WR/TE/FB/whatever. He’s not a particularly appealing prospect for those jobs. He’s strong and mobile compared to QBs, but small and slow compared to WRs and TEs. I never thought about FB but that doesn’t sound like a great fit either, or a very appealing one for him.
The broader question is complicated. Changing from cornerback to safety seems like a pretty common move, for instance. Offensive lineman get moved from one spot to another all the time, and those jobs aren’t all the same. Going from offense to defense or vice versa is less common but it definitely happens, and the posters who say it depends on the player and the position (and to some degree on the team) are right.
In fact, there are a number of examples of star cornerbacks (such as Charles Woodson) who shift to safety late in their careers, when they’ve lost a step, in order to get another year or two out of their careers.
“Jim Jebow” - 
Having never tried out for the NFL, myself, I can’t speak (or post) from personal experience. However, I’ve read a few books about the NFL in my time and one of the things I’ve gleaned is that running back is generally considered to be the easiest position to transition to when moving on to the pros. The hardest? #1 - quarterback (at least as of when I read the book that told me that which was back in the '80s) with OT and CB being #2 and #3 in some order. After that I have no idea. For “Jim Jebow” I’d consider: TE, FB (although those seem to be going out of style), and maybe LB. I think those are about his only options if QB’s not working out for him. Personally, I thinking sticking with the lecture circuit is the way to go for him since no team in the NFL seems interested in letting him try out at QB anymore.
Never mind Tebow/Jebow for a second. Is Tim Tebow the first or only successful college option quarterback who’s found himself a man without a position in the NFL? Hardly.
Many superb option quarterbacks who won national championships in college either weren’t drafted at all by an NFL team, or were late round draft choices of teams that hoped to convert them to another position. How’d that work out?
Well, former Oklahoma quarterback (and Congressman) JC Watts was drafted in the 8th round by the New York Jets, who hoped to make him a running back. He went to Canada, instead.
Scott Frost was a superb option quarterback at Nebraska, but was only a journeyman safety/nickel back and special teams player in the NFL.
Tony Rice won a national title at Notre Dame, but wasn’t drafted in the NFL.
Heisman winner Eric Crouch was drafted by the Rams who hoped to make hi ma receiver… but he took such a pounding as a receiver that he gave up that idea. He later became a safety in the European League, and a benchwarmer in the Canadian Football League.
Matt Jones, a quarterback at Arkansas, was moved to wide receiver by the Jacksonville Jaguars, but he never amounted to much.
Very few college quarterbacks succeed in the NFL at other positions.
Another one that did: Kentucky’s Randall Cobb, currently a WR with Green Bay. He did play a fair bit of WR at Kentucky too, though.