Migrate Wix sandbox site onto existing Wix domain/site

I have an existing website and one that was built on the same Wix login ready to be migrated onto an existing domain. I need it migrated over and ensure that NO CONTACTS that are on the existing domain are lost. Also, one last thing before this occurs.. I'm not sure how in the past that people were able to join our "mailing list", but I'm sure it was on the website somewhere. I'd like to have that put onto the homepage or ensure there's a contact us / join our mailing list somewhere on that page. I don't see where that is anywhere on our current site? So, adding that would be a huge help. along with the contact us page. I think that can be maybe changed and added somehow to the Contact Us page? Join our Mailing List should be on the Home Page, just under the 3 buttons.. and then maybe have that take them to the contact us page to fill out their name/email.???