I have just had to replace my old IMac. All data and applications have been mirrored onto the new machine. Acrobat Pro XI tries to start then I am redirected to subscribe to acrobat DC. Is the $750 I spent 18 months ago to purchase this software waisted and I now have to pay by the month for DC?
You can't mirror Acrobat. You have to activate it on the new machine.
However, why did you purchase an outdated application for so much money 1.5 years ago?
You can purchase the current version of Acrobat Pro (not the subscription version, but a perpetual license) for less than $500 (in the US).
The version you bought is by now outdated, not supported by Adobe, not compatible with the latest operating systems (at least on a Mac) and does not qualify for an upgrade.
Sorry, I made a mistake in the last sentence. Acrobat XI does qualify for an upgrade to Acrobat 2017.