The example I gave, Deft Strike - is a single target power that is both a range and melee power. Presuming a rogue is using a dagger, with can make both melee and range attacks - if the rogue started to make a melee attack with the dagger using Deft Strike and then had his target teleport 2 squares as an immediate interupt would you let the rogue continue his attack by throwing his dagger instead using the same Deft Strike attack (assuming the attacker still has LOS)? I can see different DM's going both yes and no on this question.
No, because he's made his attack based and modified by it being a melee attack. He hasn't taken an attack of opportunity for it being a ranged attack (which his target would have full rights to do, before the attack is made, and before they used their immediate interrupt) or taken any of the other penalties for it being a ranged attack that -would- apply in this situation. So, no, he certainly cannot change a melee attack to a ranged attack mid-execution.
Here's how it flows before the interrupts:
Begin executing power
Begin melee attack
Resolve attack roll
Resolve hit
Now, let's say for sake of argument the target bamfed away on the attack. That changes it to:
Begin executing power
Begin melee attack
Bamf away
Resolve attack roll (negated)
Resolve hit (negated)
Now, let's say you decided the attacker could switch to ranged:
Begin executing power
Begin ranged attack
Bamf away
Resolve attack roll
Resolve hit
Well, now the power is a ranged power, which provokes OAs. The target is still adjacent at the time that action begins, which is when OAs occur. This turns it into:
Attack of Opportunity
Begin Executing power
Begin Ranged attack
Bamf away
Resolve attack roll
Resolve hit.
At this point, it's rediculous, and did that actually make the game more fun? No?
It's much simpler just to say 'Look, you made a melee attack, it resolves as that.' This isn't Magic the Gathering. Trying to adjudicate it like that when it is actually a roleplaying game is failboat time.