Cannot restore, update, or unlock iPad. Now a very expensive paper weight.

Exactly the same situation here. But following Apple's *own* instructions only gets you to a dead end: "The ipad software update server could not be contacted. There is not enough memory available."

This renders the iPad useless. Apple "genius" claimed it was terminal, notwithstanding that it was Apple's problem, not ours.

A user called Norweg posted instructions on how to circumvent the problem but those did not work for me. After wasting an entire day due to Apple's total recalcitrance, including the "help" desk, I finally managed to work it out myself. You have to follow an amended version of Norweg's instructions. First, download the latest update *for your particular* ipad to your computer:

theiphonewiki dot com/wiki/Firmware/iPad/15.x

Then start up your ipad in recovery mode while connected to your computer. When prompted to press the recover button make sure to press option on your keyboard at the same time. That gets you to a window where you can choose a file. Choose the latest update which you just downloaded. Your ipad should start updating.

Once done unplug your ipad (this is very important or else it will take you to square one which happened to me several times before I figured it out) and then restart your ipad while unplugged.

Should boot up as an empty ipad.

What I'd still like to know though is why when I bought my new ipad, Apple locked me out from accessing my old ipad. That's totally unacceptable.

The warning for lack of available space likely refers to your computer - and not the iPad. Your computer needs sufficient available storage to download, unpack and verify the update - prior to installing the update on the iPad.

While you may appear to have ~400GB of unallocated space, your computer may have multiple partitions; one or other may have insufficient space.

For Windows PC, iTunes typically attempts to download and unpack software updates in the C: (System) partition.

You're not the only one seeing this. The same day that Apple released MacOS Ventura and iPadOS 16.1 (ironically the same day you made your original post), Configurator and Finder stopped being able to restore iPads. Our district can no longer wipe and restore our devices. We've tried every option at our disposal. But what works the best is literally carting in boxes of iPads to the Apple Store to have them restore them until Apple fixes this issue. I recommend that you do the same. Take the iPad to an Apple Store, tell them the situation, and have them fix the problem. That specific service is usually free (at least in my area it is).