The official rulebook states:
"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."
Replay rules have been updated as of May 2006, to bring the TCG into line with the OCG. Previously in the TCG, if a replay was triggered, and the player that was attacking chooses not to have his monster attack again, that monster could still declare an attack at another point in the Battle Phase. This is now no longer the case, if a monster chooses not to attack after a Replay has been declared, that monster cannot attack for the rest of the Battle Phase.
In addition, according to the OCGref1, ref2, if you attack with a different monster, the first monster cannot change to Defense Position during Main Phase 2 since it is still considered to have declared an attack. The same also applies monsters flipped face-down by cards like "Book of Moon" in the Battle Step in response to an attack declaration; that monster then cannot be Flip Summoned in Main Phase 2, because it is still considered to have declared an attack.
NOTE: A Replay does not occur in the following situations:
- 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 to, or removed from, the attacking player's side of the field.
A Replay also occurs when the number of monsters on the opponent's side of the field changes.
Player 1 declares an attack with "Luster Dragon", choosing Player 2's "7 Colored Fish" as the attack target. Player 2 responds to the attack with "A Rival Appears!", Summoning a "Nanobreaker" from his hand in Defense Position.
Because the number of monsters on Player 2's side of the field has changed, a Replay occurs, and Player 1 can do one of the following things:
- Continue attacking "7 Colored Fish" with "Luster Dragon".
- Change his attack target and attack "Nanobreaker" with "Luster Dragon"
- Stop the attack of "Luster Dragon".
Player 1 has a "Warrior Dai Grepher" on the field. Player 2 has a "The Six Samurai - Yaichi" and a face-down "Cunning of the Six Samurai" on the field. Player 1 attacks Player 2's "The Six Samurai - Yaichi" with their "Warrior Dai Grepher". Player 2 responds to the attack with their Set "Cunning of the Six Samurai" and Tributes their "The Six Samurai - Yaichi" to Special Summon their "The Six Samurai - Zanji" to their side of the field.
Even though the number of monsters on Player 2's side of the field has not changed overall, the number of monsters had changed in-between, giving Player 1 the chance for a replay. Player 1 can now either attack "The Six Samurai - Zanji" or stop their attack.