Gert,
The 987cX units use the same software updates as all 987 units do. There are no separate software updates for the International versus Domestic units as they all use the same ones. When you register your unit on the Humminbird web site it should be showing the 4.770 software update.
No the 987 cannot use the software for the 981 or any other unit. It will not recognize it so you cannot break anything if you accidently try and load the software for another unit.