I have used Adoibe Acrobat IX Pro for the past two years but it does not function since I upgreaded to OS Catalina yesterday. Must I start again from scratch and purchase the latest Acrobat version or is there an official upgrade procedure available to previous users as is usually the case?
I purchased this MacBook Pro with installed software and apps, 2nd-hand almost two years ago and don't have any knowledge of its software registration history.
You left it too late to upgrade at reduced price. Adobe allow upgrading for two versions, so when Acrobat 2015 and Acrobat 2017 were available you could have upgraded. This has been the policy for many years.
By the way, if you purchase a used computer with apps on it, these still belong to the original owner and you aren't allowed to use them UNLESS they complete a license transfer, and you send it to Adobe. You wouldn't have been able to use an upgrade either.