Opened image in Gimp and
duplicated layer 2 times.
(Left hand image)
Desaturated top layer.
Lightened middle layer using levels by
sliding the right hand slider to the left.
Blended middle layer with original by HSV value.
Blended top layer by Overlay.
(Middle image)
Added new layer from visible.
Blended this layer by grain merge.
(Right hand image)
This method does not blow out the highlights
the way simply lightening the image does
and brings back contrast for a true high key image.