You should prevent any content editing while this process is running.
You should NOT trust this module will migrate your data perfectly. If it fails you must restore your site from a backup. Test on a copy! Make backups!
1. Install filefield and imceimage_2_filefield modules in the normal way.
2. Optionally install imagefield module.
3. Grant "administer imceimage_2_filefield" permissions - /admin/user/permissions
4. Configure and run - admin/settings/imceimage_2_filefield
You may have to fix any references to the migrated fields
If you write an image tag in a theme, you will find that filefield paths do not contain an initial forward slash like imceimage. Use or override theme_imagefield_image()
print theme('imagefield_image', $node->field_myfield);