I don't think that the feature to switch between stencils is currently available in the product. However it will be a very useful feature, I agree!
As of now, the options that I can think of are
Option 1 -
1) Download your existing form
2) Edit the downloaded form json file and change the "stencilSetId" value to your new stencil id (SELECT * FROM STENCIL_SET)
3) Import the modified form json file and continue working on it.
Option 2- Recreate the form from new stencil
Option 3 - Modify the form JSON data directly in database and set "stencilSetId" to your new stencil id. ***THIS IS NOT A RECOMMENDED OPTION ****