let's see if I sorted out all right what was told to us:
1. OEM O2-sensors will be changed to wide band sensors
2. additional controller will be installed similar to a pc
3. ECM will be flashed with a) in tuning section only with OEM plus unknown code and b) a new base map
4. the ECM afterwards will appear and act as before and if accessed by SESPT or Digital Technician no difference will be shown.
and conclusions:
1. system stays in closed loop almost all the time and ajusts both AFR and timing on the fly
2. changes up to 20 % from the new base map are supposed to be "autotuned", mayor changes (esp. injectors) need reflashing the ECM.
3. you still may change the base map by SESPT/TTS or other device yourself if you wish - and see fit - to do so.
so after all info gathered here this seems to be the solution for everybody who has no or only mediocre tuning capability with standard tools at hand.
I may give it a try next year when factory warranty has expired. Meanwhile please post further experiences