Changing Option to Defend

[rockwoofstone]

I've searched for an answer to this, but haven't come across anything which explains it, so apologies if I've missed an existing thread.

Moves:
Player 1 moves <1/2 MA, and chooses the option to attack player 2 (with whom he is now engaged).
Player 3 moves in and also engages player 1.

We now move on to the rest of the options:
Player 3 has highest DX, so gets to go first.
Player 3 attacks player 1. Player 1 was going to attack, but now regrets that, and wants to defend instead, but it's currently player 3 who us completing their option.

Now, this is where my question comes in.
The rules say that a when it comes to a player to take the rest of their option, they can change their mind within the restriction of options they are still able to complete.
However, in this instance, player 1 isn't having their option at this point in time; it is player 3 who is having their option.
So, can player 1 change their option, even though it is player 3 who is completing his option, or is he only able to do that once he get to completing his option (and therefore cannot defend against player 3)?