Hello Matt, Ben here - your Shopify Guru!
Are both pages managed by your personal Facebook account? If so when you click on the account section in Shopify you should see a drop down box to select the other page.
If the new page is managed by another personal account you will need to disconnect your account from Shopify. You do that by using the button here:
If you do not see this try clearing your browser cache and cookies. Then logging back into your Shopify admin and try again.
I hope this helps! Feel free to reach out if you have any further questions :)
Ben C - Shopify Guru