Making significant changes to an existing store for a new theme whilst not breaking the store

[johnnyboy1981]

I'm about to start developing a custom theme and store for an existing Shopify customer. This will require some fairly substantial changes to the base Dawn theme using the Liquid templates etc.

They have an existing store but the work that is required is fairly substantial and will require lots of changes to product collections, variants, page content etc.

It kind of feels like to me they need a new store...but I'm unsure how best to do this.

Is there a way to duplicate a store with all the products, reviews, collections, orders etc to a new development store?

Obviously I am familiar with shopify-cli and being able to create development themes, but it seems like I need to make some breaking changes to their existing site in order to develop their new store.