How to Deactivate all instances of Adobe Acrobat Pro XI

I am the IT Manager for a regional scheme of a UK Charity.

We got a discounted version of Adobe Acrobat Pro XI, on a perpetual license.
Unfortunately, I did not deactivate Adobe Acrobat Pro XI  when we went through a legacy hardware replacement ( transition from Windows 7 to Windows 10 ).  

I have an email address and password from when Adobe Acrobat Pro XI  was first installed and registered, but when I try to log in using this email address, it says there is no Adobe account associated with it.

When I create a new account and try registering the serial number for the Adobe Acrobat Pro XI  product, it says it is already registered.   

I need help identifying which email address is linked to the serial number so I can go in and deactivate all instances (devices).

Regards

Iain