Hi Dieter,
Of course it depends on exactly what features of Customs Management and Risk Management you are using, and much more on how you are using them. But in general, you should not expect many issues.
GTS 10.0 is more sophisticated than GTS 8.0, but the basic functions and philosophy are very much the same. You should upgrade your Development system and then re-test everything that you normally use. Regarding the Seeburger BIS, I think you will need to install the updated mappings, but should not expect any problems in that area.
If you don't (yet) want to exploit the additional functionality, then the biggest challenge - especially for the users - will be to get used to the new navigation and screen layouts. They are much improved,but take some time to become familar.
Enjoy your journey!
Regards,
Dave