Let's say I attack with Duskwatch Recruiter, a 2/2, and someone blocks with a 1/1 token. After they choose their blockers, could you cast Waxing Moon to transform Duskwatch Recruiter to Krallenhorde Howler, and assign damage as Krallenhorde Howler, a 3/3?
Yes, there is nothing that prevents you from Transforming at anytime you could cast Waxing Moon.
There are no special timing restrictions when transforming a double faced card.
Lets break down your example step by step:
- You attack with Duskwatch Recruiter, your opponent declares a 1/1 goblin token as a blocker.
- After blockers are declared, but before combat damage is assigned, you cast Waxing Moon targeting Duskwatch Recruiter. Your opponent has no responses.
- Waxing Moon resolves, Duskwatch Recruiter transforms into Krallenhorde Howler and gains trample.
- You proceed to damage step. The goblin assigns 1 damage to Krallenhorde Howler, Krallenhorde Howler assigns 1 damage to the goblin token and 2 damage trample over onto your opponent.
Final Result: Token Dies, Krallenhorde Howler survives, Opponent takes 2 damage.
You can do what you want, but NOT during the combat damage step.
The combat phase is broken up into 5 steps:
Beginning of combat Declare attackers Declare blockers Combat damage End of combat
During combat damage step, players don't get priority until after combat damage has been dealt. That means that if you waited until combat damage step to cast Waxing Moon, it would be too late; Duskwatch Recruiter would have already dealt 2 damage.
However, during the declare blockers step, each player also gets priority, after blockers have been declared. So you can cast Waxing Moon then, which is before combat damage step. Then, during the combat damage step, each creature will deal damage equal to its power, and your Recruiter will have 3 power at that time, so it will deal 3 damage.
- Declare Blockers Step
509.1. First, the defending player declares blockers.
509.4. Fourth, the active player gets priority.
- Combat Damage Step
510.1. First, the active player announces how each attacking creature assigns its combat damage, then the defending player announces how each blocking creature assigns its combat damage.
510.2. Second, all combat damage that’s been assigned is dealt simultaneously. This turn-based action doesn’t use the stack. No player has the chance to cast spells or activate abilities between the time combat damage is assigned and the time it’s dealt.
510.3. Third, the active player gets priority.