Hello,
We’ll be able to help you with the migration from default header to header builder but you need to solve a problem with Customizer mode on your site. Once we go to Appearance > Customize or XStore > Theme Options we get 403 page https://prnt.sc/pbo6fx with the locked request, can’t build the header because of this. Could you, please, contact your hosting support to unlock customizer functionality and ask them why this request is locked? Would be grateful if you provide us their answer. We are interested to avoid this problem with other users in the future and need to study what requests and why are blocked and improve them if possible.
Regards