One of my primary keys is in the form of XXX-YY (XXX:three digit number, YY: last to year digits) which doesn't lend itself to sorting. So I am reversing it to YY-XXX via a left,right and & calculation. How do I keep these values, as values only, not as outputs of a calculation so I can delete the original pk field?

You can use Replace Field Contents on the new field or the orig field itself. Be careful to make sure the calc is working first as you cant revert those records. Also, I wanted to point out that you shouldn't be using this as a true primary key. Primary keys should be generated and not really meaningful. You should instead use the native auto serial or UUID. You can still use your number but as a second field.