Update on my situation described above: Shortly after my call to the OS SEN line my flight was indeed cancelled (noted in the App and by email). Unfortunately, online rebooking was not offered as an option. Hence, I called the OS SEN line immediately again, hung in the waiting line for quite a while, then decided to speak to an English-speaking agent. He misunderstood my booking code a couple of times and after cross-checking every possible personal information with me, he finally was able to load my booking. I asked to be rebooked on a specific flight on Sunday (day before the strike). He then told me the flight was also cancelled, as all other flights. I then asked him to check for the correct date ... And indeed he had checked for the strike day ... Then he told me that re-booking to the flight that I suggested was possible and that he will issue the new ticket. I then asked if it would be rebooked in C or in Y. He then mentioned that "they" would never downgrade customers in such a situation, but rather upgrade them, for example to Austrian First Class . This left me rather speechless and I dreamt about my VIE-FRA flight in OS F class ...
Although I was very skeptical that the rebooking/issuing of the ticket would work given the bumpy conversation with the agent, I indeed received a re-booking confirmation and flew home in OS C yesterday. Let's see how many miles I will be credited.