I have only done a few position switches and all were at D3, as I recall. I've only done it when took over a team and needed more of one position and less of another (example....needed more RBs and less WRs to run the Wishbone).
Out of those (lets say....maybe 6-8 total switches), only two worked well. First, I recruited a high Blocking TE and converted him to WR in order to increase my team blocking ratings for a 100% running wishbone. I felt like it was a good switch...but I could not really tell.....since it wasn't a focus position.
The second one.....was ABSOLUTELY STELLAR. Took over Pro Set based team and converted the best WR to RB, since I would be running the wishbone 100%. This guy was absolutely fantastic....after the switch he was actually my highest rated RB (he was a junior). I had a very good stable of RBs and he was still the best. Led the team for 2 seasons and was one of the best RBs I've ever had at D3. He had very high ELU and SPD for D3. I've since looked for other WRs to convert to RB on purpose....but have not found any WRs that had better Speed AND Elusiveness than RBs that I could recruit.