CHIN REDUCTION - will my face sag? (photos)

I had double jaw surgery and genioplasty with an amazing surgeon 4 years ago. Now my mentalis muscle is overworked/tight/ too short and the surgeon wants to revisit my genioplasty, shortening my chin. I am 40 with good skin quality. He will go through my mouth and remove a wedge of bone, fixing it with a plate. He says the muscle and skin will 'follow' the new position and be OK since I'm already straining to close my lips. I am worried about sagging. Should I be? Will I need a facelift later?

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