Changing CSC works flawlessly on this one, the usual method:
* Connect via SDB (wireless or wired connection)
* Run launch_app csc-manager.csc-pre-configuration
* Choose CSC, wait for the watch to reset, all done. SPay works as well now.
eSIM: The Gear S3 LTE models have eSIM RSP v1.x, in case anybody was wondering why LTE/3G connections were not possible in many countries (where operators are probably a bit concerned or regulated WRT security).
Galaxy Watch (well, at least my SM-R805F) supports ONLY RSP v2.x, which means it will work on most European carriers, including Telekom.de. From esim-server, in IDA:
Gear S3: std::string::string(&v8, "/gsma/rsp1/es9plus/initiateAuthentication", &v7);
Galaxy Watch: sub_64FD4(&v14, "/gsma/rsp2/es9plus/initiateAuthentication", "");
The activation of the eSIM should be done on the same CSC as the eSIM network and the mobile phone paired. Once the eSIM is provisioned and the watch has GSM connectivity, the CSC can be changed again, the eSIM profile will stay.