I start each year new, and just coach them up, when I'm the head coach, anyway.
I've seen it done differently, though.
We had a kid on 9-11 last year, and he got put at tackle, and he was terrible. I wanted to try him somewhere else, and his dad (a coach) was like, "No way, he's played tackle for five years!" I was like, "Well, nobody ever taught him anything, apparently."
We had a kid on another team at OLB, and I suggested moving him to MLB. The coaches all said, "No, he's never played anything but OLB." I said, "He's SEVEN!" They acted like he'd spent ten years in an NFL room.
I think a lot of coaches don't trust their ability to teach. They seem like they think they can, but they don't truly trust it. And a lot of them are correct not to.