A better system definition would be iMac 13 , 2 , which is supported in Sierra . If that were my system I ' d think about carving off a partition on the Yosemite drive to install Sierra as a test . I ' d start by updating the boot loader assuming you have Clover to whatever is the current release , and making sure to have a current backup of user files , or better yet a bootable system drive clone to revert to the working Yosemite install should something go wrong . If you find that the Sierra partition boots fine , then you can decide to try a direct update from Yosemite , or just erase that partition and restore everything from a Time Machine backup into a clean Sierra install and reclaim the emptied partition space .