I thought I would reply to this in case someone else searches on this topic. I want to save someone else from having the experience that I had.
I tried Regenerate Thumbnails (http://wordpress.org/plugins/regenerate-thumbnails/). This did not work. What I learned is that it will only regenerate thumbnails that are missing. (?) Apparently, anyway.
So I did some research and found this plugin – Force Regenerate Thumbnails (https://wordpress.org/plugins/force-regenerate-thumbnails/). It had pretty good ratings so I thought I would try it. I’m telling you now, do not do this. The minute I activated the plugin my images began to disappear from my site. Then nothing happened. Nothing was regenerated, but now my images were mostly gone, and the products all had broken image links, the image library had broken links, and the site was a mess. I attempted to login to my c-panel on my host, and was informed that my site had been suspended because of high cpu usage (which I assume was caused by the plugin, but also why the plugin stopped working). It took me about 24 hours to put the site back together.
I’m not sure how I should have gone about this problem, but this was not the way. Maybe deleting the thumbnails by hand through ftp, and then using regenerate thumbnails. Hopefully, I can save someone else a headache or ten. LOL.