Upgrading my Adobe Acrobat Pro XI to 2020

As @John T Smith indicated, there is no free upgrade path from Acrobat 11 to anything. Furthermore, Acrobat 11 simply does not run on any recent versions of MacOS (due to Apple's purposely making the new OS versions incompatible with existing applications and especially 32-bit applications).

Your choice is to either purchase a perpetual license for Acrobat Pro 2020 for MacOS or to subscribe to Acrobat Pro DC. (Note that you never “buy” any Adobe software itself; you buy licenses to use such software. You never own the software itself and that is true for virtually every piece of commercial software that is currently available!)

The so-called “perpetual license” is good for as long as you are using a hardware and software environment that the version was designed for; given Apple's proclivity to effectively obsolete existing applications with both operating system and hardware incompatibilites on a fairly regular basis, such “perpetual licenses” are not necessarily a very good deal.

The subscription model, although it requires a monthly payment, doesn't require any upgrade costs when a new version needs to be released to deal with OS and hardware changes.

The bottom line is that this is your choice.

To install you use the disc or your original download

There is no such thing as a free upgrade