Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to Wintermoor Commander may become lethal if Knights you control leave the battlefield during that turn
For practical reasons, WotC is reluctant to revisit unpopular planes, but just this once, it worked out

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It was these that originally convinced Nintendo to green-light English-language cards based on the Japanese video game, which had been introduced in 1996
If the target has become illegal, the ability won't resolve and none of its effects will happen