Ah yes the Terrible Horrible No Good Very Bad new selling form.
It's supposed to work better with phones, which overlooks that only a few newbie and small time sellers are using phones to list, since they are not very flexible.
There are a few workarounds.
Some have said there are three dots at the top right of the THNGVB form which allow us to switch back.
I have been using an existing listing, then Sell Similar, then when the Advanced/Business form for that turns up, changing everything possible, then uploading, then -and this is the important part- making a template of my new /older form.
We should not need to find workarounds. But here we are.