I need to create a child-theme from Twenty Seventeen on a site that has been running for a while so I can make some modifications. If I switch it to the child theme will I loose all the menus, widgets and other things set in the customizer? Are there any other issues I might run into?
Yes,the Customizer settings will not carry over from parent to child. Activating a new child theme essentially means you are starting from scratch with regards to the Customizer. Perhaps, try cloning the site and run it on a development server or local machine, and make the switch, then see what is lost.
I did find a free plugin that i tested out, and successfully exported and imported the customizer settings on the twentyseveteen theme/child theme. Super easy. Maybe give that a try. Customizer Export/Import https://wordpress.org/plugins/customizer-export-import/