If you keep making that face, I'll demote you back to just friends again.
Literal
Already, such face [object-を] do-[conditional-なら], again just [possessive-の] friend [to-に] demotion do [feminine-わよ].
格下げする ('to demote, to downgrade') — 格 'rank' + 下げ 'lowering' — applied to a relationship is playfully threatening. ただの友達 ('just/mere friends') implies the current relationship is above friendship. The conditional なら makes the demotion contingent on the face. もう ('already, geez') opens with mild exasperation. The わよ ending is assertive feminine speech.