Ser Jorah Mormont says, "If Ser Jorah Mormont is discarded from your hand, put him into shadows instead. Then, draw 1 card."
Rule by Decree says, "When revealed, the player with the most cards in hand chooses and discards cards from his hand until he has 4 cards in hand."
Now I have more than 4 cards in hand ( one is Ser Jorah Mormont), and my opponent reveals Rule by Decree. Then what happens? I will have how many cards in hand at last?
By the time Jorah's replacement effect kicks in, Rule By Decree has already resolved (that's why he was discarded). Therefore, the card draw after you discard him "into play" is drawn after the discard effect is over - and you keep the new card (as well as having Jorah in play).
Think of the replacement effect on KotS-Aeron Damphair that puts another HOLY character into play when he is killed. If Aeron dies for Valor, the character that his death puts into play gets there too late for Valar to kill it, right? It's the same situation with the card drawn after Jorah is discarded by RBD.
By the time Jorah's replacement effect kicks in, Rule By Decree has already resolved (that's why he was discarded). Therefore, the card draw after you discard him "into shadows" is drawn after the discard effect is over - and you keep the new card (as well as having Jorah in shadows).
I'm going to retire now. I've corrected ktom. 
By the time Jorah's replacement effect kicks in, Rule By Decree has already resolved (that's why he was discarded). Therefore, the card draw after you discard him "into shadows" is drawn after the discard effect is over - and you keep the new card (as well as having Jorah in shadows).
I'm going to retire now. I've corrected ktom.
Pfffft... That part was irrelevant to the question! YOUR CORRECTION WAS IRRELEVANT!!!
NYAH! HAH! HAH!
Hey, I didn't check the card text, despite knowing generally what the card did as it related to the question, so missed the detail. I'll totally give him the "win" on that one.
Hey, I didn't check the card text, despite knowing generally what the card did as it related to the question, so missed the detail. I'll totally give him the "win" on that one.
Ever the gentleman, ser.