Here's a breakdown:
Allowed changes:
* Face-up Attack Position to Face-up Defense Position: This is the most common change. Many monsters have the ability to switch between attack and defense position. This is often done for defensive purposes or to utilize effects that trigger when a monster is in defense position.
* Set Position to Face-up Attack/Defense Position: Monsters can be set face-down in the monster zone. This allows for a surprise attack or defense, and the monster can be flipped face-up during the player's turn to be used.
* Special Summon: Some monsters have effects that allow them to be Special Summoned from the hand or other zones to the field in a specific position. This can be either Attack or Defense position, depending on the effect.
Not allowed changes:
* Face-up Defense Position to Face-up Attack Position: Most monsters cannot change from defense to attack position without specific card effects that allow it.
* From the graveyard to the field: Monsters cannot be placed directly from the graveyard onto the field unless a card effect specifically allows it.
Important note: Some monsters have specific restrictions on their positioning. For example, some monsters can only be summoned in Attack position or can only be changed to defense position under certain conditions.
It's important to carefully read the card text to understand the rules and effects that apply to specific monsters.
If you have a particular scenario in mind, please provide more information, and I can help you determine if the specific change you are asking about is allowed.