Hello,
1) After migration just deactivate the theme and activate it again. Then go to WP Dashboard > Update and click Check again button. It’s important to update activation tocken. That’s all.
2) There is no need to use additional plugin (Elementor footer builder) to create footer using Elementor. Just enable Elementor for the Static blocks https://xstore.helpscoutdocs.com/article/47-static-blocks and use Static block to create footer layout. After that go to Appearance > Widgets > add Static block widget to footer widget area.
Regards