As long as the file is in the Child Theme, your changes will be preserved when updating Woocommerce or BB theme. Let us know if you need anything further!
Would I specifically need to have done something to put the file in the child theme or will the file be in the child theme as a result of me activating the BB child theme at the outset?
Unfortunately, this isn’t really Beaver Builder related as that area is being generated by WooCommerce. I’m not sure if you could customize the Woocommerce Email templates using Child Theme. Feel free to contact their support team and they should be able to assist you with the changes needed. Hope that helps and let us know how it goes!