Can HD from 2011 mini work as is in 2012 mini? My 2011 dual core i7 mini developed a logic board issue. I've acquired a quad core i7 2012 mini as replacement. The 2011 has a new 1TB SSD running Sierra OS. I plan to move the SSD to the newer mini.


Will the 2012 mini run with the 2011's SSD as is? My suspicion is no, I likely need to reinstall Sierra so the OS knows about the new hardware interface, but then I don't know how macOS works at that level. (I'm sticking with Sierra for app compatibility.)
If I need to reinstall OS Sierra on my SSD in the newer mini, can I simply install the OS "on top" of the current drive leaving data and files intact? Will that tune the OS to the new hardware and leave my user files alone? Or will it erase all but the OS (see Bonus Question)?
Or, do I need to erase the SSD and install the OS "fresh" for it to recognize there is a new hardware configuration to match? Then I'd migrate data and files from recent backup.


Bonus Question:


I have both a bootable Sierra installation thumb drive and "install macOS Sierra" app in Applications. Do those invoke different installation modes? If so, how do they differ.


Thanks for your help.
Your 2012 Mac mini should be able to boot and run off the SSD from the 2011.  Technically speaking, this is basically what Target Disk Mode is - using a different drive to boot your Mac.

There's no "tuning" of the OS nor are there any different "installation modes."