I had to do it twice. Some of the vertices had been so close, when narrowing the neck, they merged. As a result, a “flap” was introduced at the front and back seam, along the mirror axis.
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Just realized … we have a back view of the head - 
Hide (H) the face, so the front mesh isn’t in the way.
Then select all, to see what is left of the mesh to work with.
Look at the head from behind …
and adjust the mesh.
When done, to Unhide press Alt + H.
One final “Smooth Vertices” and it looks fine, at least to me.
One of those moments … LOL
Cheers,
Ray
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