There will be no grace period either if anyone doesn't have enough RB. This rule has been in place for about a decade in MFN time, everyone knows about it, wants it, so if someone only carries 3 RBs, I have no sympathy for any roster problems that come of it.
If someone doesn't want to cut players to sign a FA RB, I would suggest getting a fast TE or FB or backup QB on your roster that can run the ball in a pinch.
You can also take a WR and switch his position to RB. You won't have to wait for weight gain, that will come eventually.
I'm not trying to make this difficult at all, and while the easiest thing to do is let people do whatever they want, there is a strong majority in this league that simply do not want any WR to carry the ball.