Rotating Images with file_column

Hi everyone,
I have a photo site that I'm making and I'm using file_column to
handle the auto-generation of several "version" of each photo:
essentially thumbs and fuller-sized version. The full sized versions
retain the original's proportions, whereas the thumbs are all 4:3.
This means most of the thumbs are cropping the original. This all
works great, but I want to let the user rotate the image after it has
been uploaded and "processed".

This means I can simply rotate all the normal versions directly, but
the thumbs will have to be regenerated. How can I trigger
file_columm's auto-generation function without "reuploading" the
image? Or better yet: is there a clean way to represent the image
"versions" (which would consist of dimensions and cropping) which I
can plug into the file_column and RMagick?

file_column :image, :store_dir => :photo_store_dir, :magick => {
    :versions => {
      :small_thumb => { :crop => '1:1', :size => '40x40>' },
      :large_thumb => { :crop => '4:3', :size => '100x75>' },
      :small => { :size => '640x480>' },
      :medium => { :size => '800x600>' },
      :large => { :size => '1024x768>' }

The above code works great. I just want to be able to use
:small_thumb => { :crop => '1:1', :size => '40x40>' } in my RMagick
code in order to regenerate my thumbs. Any ideas?