Upgrade from ImageField 5.x-2.2 to 6.x-3.0-alpha4 Does not produce necessary thumbnail files

When upgrading from ImageField 5.x-2.2 to 6.x-3.0-alpha4 I ran into an odd issue in which no image was shown in the imagefield widget when editing a node for pre-existing images.

This also resulted in an inability to update alt text, titles, or descriptions. (Hitting save while editing the node had no effect after entering data into these fields.)

I found that this seems to be due to the new version of ImageField using a generated thumbnail file with a ".jpg.thumb" extension. It seems that during the upgrade process in the installer, it might be worthwhile to have it duplicate all existing images and save them with the ".jpg.thumb" extension in order to make the upgrade process more smooth. It seems unacceptable to require a user of the module to do this manually...

Workaround: When I go through and manually create new ".jpg.thumb" files for my pre-existing image fields, the problem is resolved. I can both see the image previews in the imagefield widget when editing the node, and I can adjust the alt text, titles, and descriptions without problems. (I'll probably run a batch in a graphics editor to duplicate and tack on the thumbnail extension for now.)