Time Machine wants to decrypt the backup volume I replaced the ssd in my mbp.  Rather than simply restore from time machine, I reformatted and did a fresh install of Mojave 10.14, I like to start from scratch every so often and this seemed like a good time.

The computer is backed up via an unencrypted TM backup on an encrypted backup volume, which is an HFS+ volume on a HDD connected via USB.

Now I want to preserve the old TM backup, and start a new TM backup on the same disk. I want the backup to not be encrypted, and the backup volume to remain encrypted.

So I went to TM preference pane, click "select disk", click the volume, uncheck "encrypt backups", click "use disk." The OS throws up a modal that says "The password for the disk "DiskName" is required to turn off encryption." When I fill in the text box, the button "Turn off Encryption" becomes avaialable.

Again, the existing TM backup is not encrypted.

What happens if I click Turn off Encryption? And how can I get from here to where I want to be, with a volume that IS encrypted, and a backup that IS NOT encrypted?
The thing you struggle with is that you have a misconception that the encryption of the backup is separate from the encryption of the volume - it's not.

I.e. when you do encrypted Time Machine backups, the volume is encrypted - and that's the only encryption that exists.

So either you have encrypted Time Machine backup via an encrypted volume, or you have a non-encrypted Time Machine backup on a non-encrypted volume.

All in all this means that if you click "Turn Off Encryption", you're removing the encryption on the volume, which is not what you want. You basically already have the setup you're looking for which is an encrypted volume.