Replay

After you’ve announced your attacking monster and the attack target monster during a Battle Step, the attack target might be removed from the field, or a new monster may be placed on the opponent’s side of the field before the Damage Step, due to a card’s effect.

This causes a "Replay". When this occurs, you can choose to attack with the same monster again, or choose to attack with a different monster, or choose not to attack at all.
Note that if you attack with a different monster, the first monster is still considered to have declared an attack, and it cannot attack again this turn."

A Replay does not occur in the following situations (this assumes Player 1 is attacking and Player 2 is defending):
-The ATK or DEF of a monster(s) on the field changes.
-The battle position of a monster on the field changes.
-A monster is summoned/removed to/from Player 1's side of the field.

Basic stuff, learnt.

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