The design change started in 09/80, for the '81 model year .
The part numbers are different .
The differences that I know are : 2mm larger valves, for the US market, a 'fresh air' system was added, air was drawn into the exhaust port, to continue burning any left over fuel .
The earlier heads had problems with valve recession, the valve seat alloy was changed starting with bikes produced 09/80 .
The new alloy had the property of not transferring heat from the valve to the cylinder head .
A phenomenon called 'valve face plastic deformation', in essence, the valve overheated, and started flowing around .
I have one of the first bikes with the new seats, and my exhaust valves were worn out at 45,000 miles, the valve face metal 'flowed' toward the valve stem .
When I had the heads reworked, I was told that at best I had 5,000 miles before the valve would have pulled into the port area by the valve springs .
If you're wondering if the heads are interchangeable, I do believe they are .
