Mac OS upgrade

One can still use the computer as is or another option would Trade It In with Apple. Depending on you location, Apple does offer a Trade In against the Purchase of a New Computer.

Example, I am located in Canada and on the Apple Canadian Site you just needs to input the requested details of the computer. Right there on-line they offer what they see as a value to the computer. The reminder of the process is staring forwards and presto the cost of the new computer minus the value of the old computer.

Personally, I have used this method three times this year to purchase two new computer and one new iPhone. Works very well.

Food for thought.

No.

High Sierra is the version just before Mojave.

If the computer does not qualify for Mojave - it will not qualify to run Catalina, Big Sur, Monterey.