Yah even if it hasn't happened to you yet, be aware that this is something that happens. There actually is a table of firmware versions, etc - once you upgrade

Yah even if it hasn't happened to you yet, be aware that this is something that happens. There actually is a table of firmware versions, etc - once you upgrade your old Mac hardware past a certain firmware, you will no longer be able to run certain versions of macOS "before" the firware was supported. This isn't just between "major" versions, also between "minor" versions of macOS For example, between the current version of macOS Sierra, which is on 10.12.6 (6 being the minor version), if you try to install for example 10.12.x where x is below 6, etc - you might get the infamous ? or the circle with a slash through it on bootup.