In the file below, I simply created a sphere with a R100 centered at 0, in the Front view rotated it with the Gumball -90° so the seam is on the bottom, drew a plane in the Top view (on World XY) and trimmed off the bottom half of the sphere. So far no problem.
Then I tried ShrinkTrimmedSurfaceToEdge to get rid of the trimmed off part and be left with an untrimmed the half-sphere. No go. Tells me the surface is already shrunk and that it’s still a trimmed surface; if I untrim the two edges, I get the sphere back.
Methinks this should work and the result should be like the object on the ‘Revolve’ layer. Note also that if the splitting plane is slightly above the world XY - i.e. no longer on the ‘equator’ - Shrink works. But not if it is below - normal, because there is a piece of the seam left then. So the exact half-sphere looks like a ‘special case’.
HemisphereNoShrink.3dm (452.1 KB)