There are ways to get the older 10.10 and 10.11 versions installed on that mac. Apple and/or Heidelberg may give you some ideas on this.
I remember going from 10.6.8 to 10.8.5, then to 10.10.5....6 to 8 was chargeable?, after that it was 'free'
I always make a bootable USB image of the OS's for...