Only creatures controlled by a battle's protector can block creatures that are attacking that battle. This means a Siege's controller can never assign creatures to block for it.
Invasion of Kamigawa // Rooftop Saboteurs
If a battle that's being attacked somehow stops being a battle, it is removed from combat. Similarly, if its controller changes in the middle of combat, it is removed from combat.
If a non-battle permanent that is already on the battlefield become a copy of a Siege, its controller chooses one of their opponents to be that battle's protector. However, it will most likely be put into its owner's graveyard because it has no defense counters (see below).
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Kekai Kotaki
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