If I use a replacement effect like Unlicensed Armory or Zhonya's Hourglass to save a unit I'm killing to pay for Heedless Resurrection's cost, does the spell still work?

Ruling: No. While the cost is technically paid (per Rule 357.2.a, replacement effects still satisfy cost payment), Heedless Resurrection will fail to resurrect anything when it resolves. The spell references "the killed unit" to determine the Energy and Power limits for what you can play from trash. Since no unit was actually killed (it was saved instead), this reference returns a null value, not zero. You cannot compare unit costs against null, so no unit can be selected from your trash, even a zero-cost unit.

Sequence:

• You play Heedless Resurrection and attempt to kill a unit as its additional cost
• A replacement effect (Unlicensed Armory, Zhonya's Hourglass, etc.) saves the unit by recalling it instead
• The cost is considered paid and the spell goes on the chain
• When the spell resolves, it cannot find "the killed unit" to reference
• The Energy and Power comparison returns null
• The spell resolves but does nothing