Same here.  I put off "giving it a try" like all the alert messages and email marketing messages said, but finally decided to "give it a try."  It's not a try if you have no option to back out! 

I can't exit the store set-up pages, and therefore can't get to my website to edit it!  What's worse is that we're stuck in this tutorial for three months until it expires... OR overwrite our entire website with a blank Square store page.  I can't go in to backup my site in preparation because, again, I can't get to the site editor. 

If I chose to publish the trial store to my current domain, I get this message: 

"Unpublish old & publish new site: Your existing Weebly site will be replaced, but your paid plan is still active. You can bring your old site back at any time, but only under a different domain name." 

Ok, DON'T want that!!

(Two other options, buying domain or importing one I own elsewhere, don't apply)

So I chose "Use a Square Subdomain" it says "change" inside the button - did this seriously just permanently change my account to be associated with this free Square address, instead of the domain that I own??

And even after trying that, I STILL can't get to an editor for my original website!!

I've sent an email message to support and chatted with the web bot (not helpful), so the only option is to wait until someone responds to this.  Not cool.  (Time to start praying it's not completely gone...)