A symbol library has two audiences: the person choosing a symbol and the person reading it on the drawing. Decide which appearance you need to change before editing a master.

Use your own editable custom stencil in desktop Visio for Windows and retain its original copy. Choose a practice master that you created yourself. Record one intended thumbnail change and one intended drawing change so the two experiments have separate acceptance criteria.

Choose the editor for the intended result

If necessary, right-click the stencil title and choose Edit Stencil. Right-click the master and open Edit Master. Edit Icon Image changes the stencil thumbnail; Edit Master Shape changes the shape used for new drawing instances. Close the editing window and confirm the update when prompted.

First make a recognisable icon-only change and drop a fresh instance onto the practice page. Compare the stencil image with the placed geometry. Then edit the master shape itself and drop another fresh instance to check that separate change.

Do not use a previously placed shape as the sole witness for this exercise. The question is what a newly created instance gets from the master, not every possible update relationship in an existing document.

Review the library as a user would

Ask whether the thumbnail still helps someone select the right symbol. A deliberately unusual test mark can prove that the intended editor was used, but it should not remain in the working library unless it serves the design.

For the drawing appearance, compare the new instance with the agreed visual requirement at a normal viewing size. Record the custom stencil and master name beside the accepted example so a later editor knows which library entry produced it.

Restore the original or save the accepted custom version under a deliberate name. Keep the test page separate from issued drawings. The finished change should make its purpose clear both in the stencil and on the canvas.

Sources: Microsoft official guidance.