I couldn't find an answer to this by reading rule books or searching forums.

If an attack is cancelled, the attacker:

A. chooses a different target without losing the opportunity to attack.

B. moves on to the next step of its activation, such as attacking from a different hull zone or Execute Maneuver.

B.

The attack ends.

It’s not “cancelled, time is turned back and you start again afresh.”

Remember, once you even declare a target you are working your way through the attack flow and irrevocable decisions have been made.

Dras has it right.

In practice, it's usually obvious if an attack would be canceled due to not having enough dice from the get-go and your opponent will usually let you take it back.