I can usually untrim things by deleting surfaces, or in the case of through-holes, deleting the bore surface, then the edges where they intersect the parent surface. In this case, I can't do the second step on this hole (the remaining edge will not delete).

It was originally a circle subtracted from the solid. Please see highlighted yellow bore in attached image. The edge is split into three segments on that surface, if that matters.

Thank you for any guidance!