To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as an overload cost), add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
If you are instructed to cast a spell with overload "without paying its mana cost," you can't choose to pay its overload cost instead.
If you don't pay the overload cost of a spell with overload, that spell will have a single target. If you pay the overload cost, the spell won't have any targets.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Suzanne Helmigh
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Because a spell with overload doesn't target when its overload cost is paid, it may affect permanents with hexproof or with protection from the appropriate color.
The affected creatures don't lose any abilities when they become Wurms.
Effects that modify the affected creatures' power or toughness (such as the effects of Force of Virtue or Giant Growth) will apply no matter when they started to take effect. The same is true for counters that change the creatures' power or toughness (such as +1/+1 counters).
Scale Up overwrites all previous effects that set the affected creatures' color, creature types, power, and/or toughness to specific values. Other effects that set these characteristics to specific values that start to apply after Scale Up resolves will overwrite that part of the effect.
