If you were to switch the 18M Ted Williams for the 4M Ted Williams, you would need to ALSO switch every player you selected before Ted Williams to have a lower salary than the exact cost of the 4M Ted Williams. Since it's likely Ted Williams wouldn't be drafted until late in the draft, the penalty would be the quality of your team.
I see your point though, if you just take 18M Ted Williams in Round 1 and then switch him before Round 2, that's a loophole. Hadn't thought about that. Let's do this. If the round we are in is x, you get 25 - x penalty points if you have to make a switch.
So, making a switch of the player you picked in Round 1 would give you 24 penalty points. There's some good faith here not to do this in the later rounds, even when it costs fewer points. I'll flag anything suspicious.