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Let’s say the upgraded character normally has 10 health. It gets upgraded with Minor Medic Kit, then gets 10 damage counters. If Minor Medic Kit is scrapped, what happens?

Any time the game notices a character with a number of damage counters equal to or greater than its current health, the character is KO’d. So your character is KO’d. Really sorry.
Source: Wizards of the Coast (2019-11-08)    【Link to Ruling】
Tags: Minor Medic Kit