I know this has been a while, but I found this page looking for the same answer. Once I discovered the solution, I thought I would share it here to help you or someone in the future.
Once you convert the classic to blocks, you can multi select the images, and then the convert button allows you to convert to a gallery.
I noticed mine stayed at thumbnail until I go to edit the gallery. I don't make any changes just save gallery, and then the images don't default to the thumbnail size any longer. This part may vary by user depending on how the images were initially added.
Hope this helps.