Train # changed on DB ticket from Nürnberg Hbf to Passua Hbf

I purchased 4 adult train tickets w/seats from Nürnberg to Passau on ICE train 23 travel date September 30 on DB Navigator app. I received an email stating “timetable change for your journey to Passau on 30 September 2023: Different train. We use a different train. Your connection from Nürnberg Hbf to Passau Hbf will be carried out by ICE 2523. If you booked a reservation you can book yourself a new seat - if available. You will be refunded the cost of the first reservation you did not use. To request your refund, please either contact your point of sale or write informally to the Passenger Rights Service Centre in 60647 Frankfurt (M). Alternatively you can use the digital rights application. Sincerely yours, Deutsche Bahn”. My questions are can I just buy 4 seats on the new ICE train (via DB Navigator) or do I need to repurchase tickets plus seats and request refund on tickets? Are my original tickets good, but without seats? What would be the best route in getting a refund? Thanks for any help!