Your base system is Ubuntu 26.04 LTS, and that allows you to add the optional Pro which comes from the company Canonical, but it may not cover all your [installed] packages; firstly as it concentrates on the GNOME desktop used by Ubuntu Desktop, and what was released by Ubuntu Unity only offered 9 months of guaranteed support; though they did state
Even though it is not LTS officially we will support it like it is LTS release
That’s been offered before by a flavor (Ubuntu Studio 18.04 wasn’t an LTS), and they provided their additional support via PPA, which may have added more complications in regards adding Pro; you may have this too with your Ubuntu Unity, though Ubuntu Unity may actually SRU fixes (this can be done for the life of standard support) and not use PPA; I don’t know (yet).
Rather than years of support; I’d also consider what is actually covered in that support (also don’t forget when 14.04, 16.04, 18.04, & 20.04 reached EOSS there were security notices warning some deb packages would only get support when switched to snap; we have no idea what that will be when 26.04 reaches EOSS as that’s still years in the future).