Attachment_Fu : how to remove thumbnails only and regenerate

hello guys

i am using attachment_fu for product images
i have hundreds of products and their images
with thumbnails :thumbnails => { :thumb => '100x100>' }

now we have decided to have thumbnails of size 50x50> only.

how do i destroy the existing thumbnails for images
and regenerate the new thumbnails

i shud be doing something like this

product_images.find(:all)
for each prod image
destroy the thumnnail. ---what method
create new thumbnail from existing images for products ---what method
end

pls help

jags