Hi Wolfram,
It seems that you can do free-running (500kHz) frequency and synchronize to external clock with higher frequency. You can do that without having to disable/enable or power cycle the chip. If you are in synchronization mode (RT pulled down with a 1k resistor) and sync signal is not present after initialization, the part will switch at the default 500kHz clock.
The restriction described in the datasheet is that you cannot adjust the free-running frequency AND be able to use the synchronization function at the same time.
It seems that you would like to have free-running frequency at higher than the default 500kHz and also be able to synchronize to even higher external clock. Let me know if I am understanding you correctly.
I would like to double check with the designer of the chip if there are exceptions to this restriction and get back to you later today or early tomorrow.
Regards,
Denislav