That won't propagate to the existing shapes on a drawing. The 1st time a shape is dragged to a drawing page, Visio makes a "local" copy of it, in the document stencil. Then, subsequent drag n' drops from the main stencil will actually invoke placement of the "copy" from the document stencil. Updating the master stencil does not automatically update these "copies". At least, not to my knowledge and experience. You either have to edit the document stencil copies, or give your master a slightly new name so Visio thinks it'll be a new, 1st time placement. Actually, that's a good practice as it allows you to go back to a previous version. Updating master shapes, especially if they're shared, is always problematical as the update may (can) propagate into older drawings and change those drawings in an undesirable way.
To update your drawing, you must update the document stencil. There may be multiple copies here. The original from the master stencil, and then if a shape on a drawing page was copied and placed, an incremented version is added to the document stencil. So, you might find shape, shape.10, shape.21. All three basically the same shape. Fun!
HTH
Wapperdude