Can I change the date of an OBB train ticket ?

Did you buy it on the official website? If so, it should be okay. If it's a discounted ticket, it might be a problem.

I bought a special ticket from Salzburg to Vienna, which was printed as a PDF. When I heard at the train station that this train would be delayed, I went to the ticket office to inquire, and was told that it could not be changed. It was interesting that this train was later cancelled, so we naturally took another train. When the conductor checked the tickets, there was no problem. Unexpectedly, the train broke down halfway, leaving us at a small station and going back to Salzburg on its own. We waited for more than an hour at the small station, and finally waited for the train to Vienna.

Specially priced tickets cannot be changed, and PDF ticket collection cannot be changed. However, you can actually go directly on short lines, and there may not be anyone checking tickets. The same starting station should be able to take different time shifts, just ask the ticket service desk when you get there, or if you encounter someone checking tickets, tell him you mistook the time and train number, anyway there are empty seats. If the starting station is different, you should buy another ticket.