I went pretty deep on this previously, specifically looking at two-star receivers moved to the secondary.
https://forums.operationsports.com/f...post2050979531
Since then, I can anecdotally confirm changing positions is pretty effective in general. In my current primary dynasty, I have two corners who came in as three-star running backs, currently both rated 88 overall as redshirt sophomores.
As you note, quarterback is an exception. If a guy can't throw, he can't throw. That isn't ever likely to change. I'm glad for that, too. It's realistic. You can't just take anyone and put him at quarterback and expect good results. (That said, I do wish way more players had high throw power. In real life, I'd guess a significant portion of college teams have linemen who can throw the ball a mile. Strong arms aren't THAT rare among non-QBs.)
A few other areas where it doesn't really work in my experience:
DL to OL: This one's a bit iffy, because it's really only one attribute that's affected. With every DL I've tried moving to OL, they end up as a power archetype. And power archetype OL do not ever progress pass block finesse. It's an error on their ratings page. If you look at any power OL's pass blocking skill group, you'll see pass block power listed twice. So PBF just never improves. Your DL-turned-OL will be stuck with a horrendous rating in that stat forever, which would make him struggle against finesse moves.
Any player to OLB: As you note, this often causes a bug. The issue is when the converted player is listed as a speed rusher. That's from Madden. The speed rusher archetype doesn't exist in this game, except when it's bugged. So their ratings page is just blank. And as you note, the fix is to change them via the edit function on their player card. Moving ROLB to LOLB typically addresses it. That issue aside, this often seems like a pretty viable move for players with the requisite physical traits.
Moving players to hybrid safety: Not really an issue specific to position changes so much as it's an issue with hybrid safeties in general, but they're significantly slower to progress than the other archetypes. If you have a guy you want to be a safety but it's giving you the hybrid archetype, just don't bother. He won't get better quickly enough to be a factor on the field. You're better off recruiting a natural safety. (Or, as a workaround, you can edit his zone coverage up a bunch before making the position switch so he comes in as a zone archetype, then edit it back down to where it was after moving him to safety.)
Power rushing OLB to DE: This works really well, but with a caveat: If you have an OLB with the quick jump trait, you probably should leave him at OLB. He'll lose that trait when converting to DE most likely. I've had a few OLB that I wanted to move to DE, but I also didn't want to lose the trait, so I just play them out of position instead. That trait is worth preserving. This position switch is mostly about aesthetics anyway; in real life, a lot of teams treat OLB as DE and vice versa anyway. EA needs to get on the "Edge" position bandwagon already.
DB to HB: I have a sample size of just one on this, but I tried converting a safety to running back. Mostly out of curiosity. His break tackle rating topped out at 39. Everything else looks good, though. Maybe it's just this one guy, but I'm probably not going to try this experiment again. (That said, the idea of an athletic running back who gets tackled when a defender breathes near him is kinda realistic, so I'm not mad about it.)
And a couple position changes that can be really fun. Neither is unusual (most players have probably considered it already), but both are fun:
WR to HB: You can end up with some pretty unique running backs this way. A fast deep threat WR might never be a good all-around running back, but he can be extremely fun as a third-down back. Especially useful for guys who have low skill caps at receiver. They'll probably be one-trick ponies at either position, but with position changes, you can often get two distinct one-trick ponies on the field at the same time. And who doesn't love ponies?
TE to FB: Again, a pretty obvious move. But a possession tight end on the smaller side can quickly become a Kyle Juszczyk clone. There are a few plays in my playbook where I can hot route the fullback to a backside wheel on a playaction, which for me translates to a 30+ yard reception at least once every couple games. Not earth shattering by any means, but if you run the ball enough to use a fullback on the field, a converted tight end will be a massive upgrade all around (especially because the game seemingly does not generate any fullbacks above three stars).
I've also dabbled a bit with moving OL to DT, which you mentioned. I have a side dynasty where I have Hawaii running a 3-4. The secondary is 100% players recruited as receivers or running backs. Half the DL are former OL. And then I cheat with recruiting so they sign elite linebackers (take control over other teams offering the players I want, remove the offer, repeat until they sign; I'm mostly just recruiting for this team so my main team has a strong conference opponent, so cheating doesn't bother me). Their current best DL is a redshirt sophomore converted OL: 85 overall as a DE. He's close to maxed out there, but given the state of that team when I took control, they're sure as hell weren't going to have an 85-overall sophomore defender anytime soon otherwise. The other OL I've done this with for them, however, have lagged in development.