I disagree on converting tackles to guards. Those are two different body types and skill sets. Guards are more squatty and have to be able to operate in small spaces.
Well,, I used to agree with your thinking that converting tackles to guards was difficult. But, I saw something this year that totally convinced me otherwise.
Bengal's LT Andrew Whitworth,, a very solid LT that is one of the better tackles in the league every season,, moved to Guard for a few games this past season. This move was NOT made because Whitworth was not performing well. It was made because of decimating injuries to their OL, and they had to insert a young tackle Collins, who could really only play tackle position.
Anyway, the few games Whitworth played Guard, he was an absolute beast. In the run game, he was pulling very well and was basically a stud "Road Grader." It actually looked like he really enjoyed playing that Guard position.