You may find the following plugins useful while migrating your theme.
- Regenerate Thumbnail :- Allows regenerating post thumbnails/featured images for the current theme needs.
- Widget Importer & Exporter :- Allows exporting/importing widgets
- Customizer Exporter & Importer :- Allows importing/exporting customizer settings.
- Simple Image Sizes :- Fine grained control of featured image and other image sizes.
- Duplicate Post:– Easily duplicate any post/page or custom post type.