What does that mean if you pay for a $2400 fare, but have to change it and the fare for the new dates is $2200, UA won't give you a credit for the $200 difference or the cash?
That is precisely what it means.
How much more was the refundable fare though over the nonrefundable?
In my case, about $150 RT.
Until / unless UA drops this ridiculous policy -- one their competition doesn't have, FWIW -- I highly recommend that everyone check the fare rules carefully. In this case, you're looking for the word 'residual,' which tells you what willl happen if the new fare is less expensive. 'ignored' is bad -- that means UA keeps it. 'refunded' or 'E.M.D. issued' or something similar is good -- it means that you'd be able to access the funds.
Also -- I doubt they're currently being offered, but for a while, LH group had some no-changes-allowed P fares (think of it as Basic Business). So, I always watched out for those also. I never saw them on the JV fares (i.e., USA to Europe), but I always checked just to be safe.
Also the other question is, what would happen if the country bars Americans for tourism but UA or the *A carriers still operate the flight? Presumably they wouldn't give you a refund nor maybe flight credits either? They'd let you reschedule without a change fee for some period of time?
Correct -- for a non-refundable fare, all you could do is (a) hope for a schedule change and then (b) if no schedule change, cancel for Future Flight Credit, subject to the caveats above regarding residual value.