Maintaining Strong Rankings When Starting New Shopify Site

We have a couple of company sites selling many of the same products and plan to migrate all onto one site. All sites currently ave very strong/high rankings. We have created our Shopify site because we need a better tool, but have not gone live yet. What is the best practice to obtain high rankings with our new Shopify site, but not at the expense of losing rankings and customers from our current sites with another host?

Are you merging multiple sites into one? Or keeping them on separate domains, just moving them both onto Shopify?