Ren here from the Shopify support team. No worries at all, here are the steps you can take to switch back. I will also outline them for you below:
You'll need to do this from the POS app directly.
Let me know if I can help clarify any of that for you!
I am currently running 3 pop up stores on classic as the new pos doesnt fullfil my needs .... but the option to switch back has been removed? what the hell is going on...if this has occured you have put my whole business at risk