The supplement is not connected to a seat reservation.
If you already did buy a seat reservation then you just need the supplement. This is 15 € 1st class. But it would have included a free seat reservation (so your money already paid for a reservation is lost). So you may also buy the suplement online with a new reservation or without a reservation and use the old one. But without this supplement you are not holding a valid ticket.
It’s possible to buy it online (same ÖBB ticket shop you did use but add Interrail as discount and searching for tickets; recommended) or at the station. When going without it on the train, it will get more expensive (20 €).
But there is information missing. Are you going on a direct ÖBB EC train from München (just for this one you’d need a supplement)?
Thank you for your reply. I am not sure if I want to make a new reservation. If I am going to pay the supplement at the station, how would I do this? Can I do it in Munich, or is it at a designated station?
I am also going home on the same route again from Rome to Munich. Is it also required to pay the supplement when entering Austria again?
Thank you for your reply. I am not sure if I want to make a new reservation. If I am going to pay the supplement at the station, how would I do this? Can I do it in Munich, or is it at a designated station?
I am also going home on the same route again from Rome to Munich. Is it also required to pay the supplement when entering Austria again?
As written before: The reservation is free of charge so it doesn’t matter. It’s no problem to buy the supplement online, too. In theory every ticket counter can sell it if they know how to do it. Should be no problem in München but may take some time. I recommend to buy it online; but it’s not needed.
If you do enter Austria on a direct DB-ÖBB EC / RJ train via Brenner: yes, it is. But not on other trains via Brenner (needs a change of trains there) and not on other routes (e.g. on the direct Roma - München nighttrain).
OK. Thank your for responding. I think I will make a compromise. My ÖBB resevations also covers the Hamburg-Munich part of the journey with the High Speed Train. Perhaps I could book a new resevation from Munich to Venice and use the old one on the high speed portion in Germany. That is, if it saves me more time than booking a completely new reservation on the whole route...
It's totally possible to buy just the supplement at the ÖBB online ticket shop and keep all the reservations.