Acrobat Pro 9 to Acrobat Pro 2020 one-off perpetual license

Not possible. Your version is way too old to qualify for an upgrade. Typically, only the last one or two versions do, and yours is about 6-7 versions behind. You need to get a new full license.

Last I heard, it was too late to get a discount on upgrading from Acrobat 9, 10, 11 and 2015. Full price only, or a subscription. 

No one here will really know the answer to that. You will have to contact Adobe directly.

As far as I know, you can only purchase such upgrades through resellers, so my best advice is simply do an internet Search for "Acrobat 2020 Upgrade".(edit: I just did and found a few places... cost was about 50% for a license for a qualifying upgrade)

As far as downloading v9, if that is how you purchased it originally, do you currently have a download link in your account page? If so, you might want to download the installer now and store it somewhere (If you got it on CD, there will be no download link). Regardless, it's highly unlikely you will be able to reinstall it on anything new, hardware-wise.