I can't imagine a process that wouldn't result in destruction of the stencil. If it were me, I'd ask a professional art restorer before I attempted such a task.
Frankly, unless the wall is coming down or need some work, don't remove the stencil art.
Furthermore, it depends on what you call 'worth'. A question for you: is the stencil art right now worth anything? What determined the value of the stencil?
There might not be a collector's market for a photographic of the stencil art; however, as a photographer, I've taken many good images which represented the sculptures and/or art work for artists to document their work. their desire was to use it in their portfolio