How to relaunch a website without affecting SEO or credibility?

What do I do in this situation when I want to relaunch my website?
KEY WORRIES:

  1. How will my google search rankings be affected?
  2. What is the downside / recommended path.

I have my website www.terreamer.com.au which has been on Shopify for 5 years.
I want to redo my website from scratch because originally we had a designer in-house and she designed every webpage from scratch (in PSD and Indesign) and it didn’t follow a template which causes problems when I want to update it.

I want to start it from scratch because

  1. I want to be able to use a template (especially a Shopify 2.0 template).
  2. Get rid of Ghost code on the website.
  3. Have more control over it with a brand new shopify template.

Is there a way to update my website and not have any problems in regards to SEO, credibility?
Is there an advised path? I was going to create a new Shopify account and purchase a template (2.0) and redesign my website.
then once it’s finish swap over the details to the new website.

Are there any issues with my approach?

Thanks Lads/Gals