See the announcement and reasoning here.

Q: Can I activate Kindly Stranger‘s ability, hold priority, then activate it again so it will transform into Demon-Possessed Witch, kill a creature, then transform back into Kindly Stranger?

[expand title=”Answer Here”]You can double activate the ability the way you described, but you won’t be satisfied with the result. We have a new rule that says if an ability wants to transform a double-faced card, but that permanent has transformed since that ability was put onto the stack, the instruction to transform the double-faced card is ignored [CR 701.25f].[/expand]

Q: Can I Abrupt Decay a Perfected Form?

[expand title=”Answer Here”]No. The converted mana cost of a double-faced permanent’s back face is now calculated as though it had the mana cost of its front face [CR 202.3b]. Even though you can’t see it without turning the card over, Perfected Form’s converted mana cost is 4.[/expand]

Q: What if I Clone a Perfected Form, can I Abrupt Decay that?

[expand title=”Answer Here”]Yes. Clone isn’t a double-faced card, so its converted mana cost is converted the old-fashioned way: by looking at its mana cost [CR 202.3a]. Except that Perfected Form doesn’t have one. The converted mana cost of this Clone is therefore zero.[/expand]

Q: If my Perfected Form is enchanted with Pendrell Flux, how much do I have to pay to keep it around?

[expand title=”Answer Here”]Unfortunately, you can’t keep it around. This is where the rules for this get kind of weird. Perfected Form’s converted mana cost is 4 because there’s a rule that says when you’re determining the converted mana cost of the back face of a double-faced permanent, you calculate it using the front face’s mana cost. There’s no similar provision for determining the mana cost of a double-faced permanent, so Perfected Form just doesn’t have one. Because it doesn’t have a mana cost, paying that cost is an impossible action, and you’re forced to sacrifice it [CR 117.6].[/expand]

Q: Amy casts Altered Ego with X=5 and copies an Apocalypse Hydra. What is Altered Ego’s p/t when it enters the battlefield?

[expand title=”Answer Here”]As Altered Ego resolves, the game checks to see if there are any replacement effects that need to occur. Altered Ego’s effect applies here and makes it a copy of Apocalypse Hydra and puts 5 counters on it. Then, the game checks for replacement effects again [CR 616.1e]. Altered Ego/Apocalypse Hydra’s new ability now wants to put X counters on it. The value of X is usually the same throughout a card, but in this case, because Altered Ego gained this ability, the value of X is 0 [CR 107.3h]. Altered Ego enters with only 5 counters.[/expand]

You can see other articles in this series here.