Coming back to my OP as i finally managed to get a data point on actually using the waiver to change flights.

To reiterate, my experience in what showed up on the app and website were identical to previous posters, as it showed a note "Courtesy Policy for Changes" both on our outbound (on Sep.21) and inbound flights (Sep.23) indicating that changes were possible, however on the app/website it let us only make changes to the inbound flight. Our flights did not start nor end up in Vienna, but rather were international flights with both LH and OS operated sectors connecting through Vienna in both directions.

What worked was a phone call to the regular customer service number of Austrian in Austria. They knew immediately about the waiver and the dates it covered. Basically after checking that our outbound flight fell inside the waiver date (on/before Sep.22), the agent asked which flights we would like to change and when we would like to fly (within the waiver dates, I presume).

As we only wanted to change our return flights on Sep.23, I gave the agent the flights details segment by segment and they were reserved as requested. We were put on hold for a few minutes after having given the new flight details, but when the agent returned he told that the flights are now booked and repeated the itinerary. He did advise that we will receive an email when the new ticket with updated flights has been issued, which for us only took place the next day as I called late in the evening Vienna time.

For anyone interested in details, according to ITA matrix the cheapest fare class available in economy for our new, updated itinerary was in fare class B. However, as per OS designation of its fares, our return tickets ended up staying as "Economy Light" fares, just as booked originally.

Hope this helps anyone needing or wishing to make a change in their ticket falling within the waiver conditions, however you may have to call Austrian directly to get the flights you want.