During the session, that's a big NO, and the game stops, period, until the book is put away.
Quoting the MM at the table in regards to Monsters that you haven't made knowledge checks about means that they suddenly gain new and interesting powers.
To quote a very recent e-mail to my GM friend;
"I've also been known to change monsters on the fly because of some annoying know-it-all. After all, if NO ONE in your part of the world has ever seen a Rakshasa, there's no flippin' way you know it's weaknesses. So if you start acting like you do, then I'm changing them. Blessed crossbow bolt? Nah, whatever gave you that idea? You need a copper rune inscripted long dagger..."
Basically, in this case, if someone started telling everyone that Rakshasa had DR 15/Good and Piercing without any in-game knowledge, they'd have to learn the hard way that the minute they said it, it was changed to DR 15/Chaos and Slashing (or Copper, just for fun).