Login to your existing Merchant Center Account with your Google account and navigate to Tools > Business Information > Website
You should change the Website URL to any other domain you have. (ie. yourstorename.myshopify.com or yourstorename.mybigcommerce.com).
Once you have changed your Website URL in your existing Google Merchant Center (GMC) Account, you can now claim your Website URL in your new GMC Account. Go to your new GMC Account and navigate to Tools > Business Information > Website.
Click on CLAIM URL.
Don't be startled when you see a yellow warning saying there exists 1 other Merchant Center account with a website that conflicts with the URL you're trying to claim. Go ahead and click CLAIM URL again.