Sorry but there is not straight way. If say you have 2 stacks "jon" and "john" wherein "jon" was mistakenly created and has less no. of images on it, you can go into this stack and click on the name field (just below the face rectangle on the image), and type "john".
Another way out is, select all images in "jon" stack and right click >> add a person >> "john". This will bring all these images to "john" stack too. Remove "jon" stack now and run FR on the images with "john" tag. To filter these images you may use Find >> Advanced Search >> and tick the checkbox corresponding to "john" tag in people panel of the advanced search widget.