I did it now and it worked immediately. My TDS744a with firmware version 1.1e is now a TDS784a. However, in the first test, signals recorded at 4GS / s showed strong distortions, switched to 2GS / s (two channels turned on), everything was normal. I then ran SPC, which ended with "pass". After that everything was back to normal, the distortions were gone. The bandwidth has doubled and it looks quite accurate even without calibration.