There's 2 really important things that need to happen for this to not be a complete clownshow of a change.
1) AUTOFILL PARITY. If my jungler is an autofilled support main, the enemy jungler should at least be filled. None of this, our jungler is a filled support meanwhile their support is a filled mid type of thing.
2) NO FIRST TIMING IN RANKED. I get the argument of "but then people might get forced into bad / off meta picks if they are filled!", so either this would just not apply if you're filled, or better fill parity would help address this. And if the issue is with one tricks, they're probably going to grief the game regardless of what they pick if they're banned out (and we can't see that and dodge them now), so if we can't dodge that free loss, then riot should literally just force people to have played at least 20 Champs once before in normals so it's impossible they'll have to first time anything. This would pretty much just effect smurfs (oh the horror), and people who have literally never tried any champions ever since starting the game (which is a very small minority). And if you're still going to complain about having to play 1 game on a few different Champs when you're learning the game, why not complain that you also have to level up to level 30 at that point?