It’s possible.

It’s classified as connection. The waiting time if the train from Austria is arriving delayed in Břeclav is 15 minutes.

You can't change your reserved seats anyway.

But there's nothing to worry about. It's a short hop from one train to the next. 

Hi Jon,

To be honest, but this isn’t the connection I would have booked, as there are many direct trains between Wien Hbf and Praha hl.n. which do not require making any station changes, however tight (most of these are Railjet trains leaving Vienna at ten minutes past the hour on every odd hour).

You can’t amend or refund a seat reservation made with DB unfortunately. The Czech operator CD is better for this and their reservation costs are also lower (app is called Muj Vlak).

Three minutes sounds a bit tight for a connection made on a fairly long-distance journey; you might make it if both trains are bang on time and you cross the station at Breclav quickly and without incident. But if you miss it, most trains in the Czech Republic do not require reservations; so you can hop on a later train from Breclav. There are exceptions to this rule, and those are the private operators Regiojet and Leo Express, as well as SC (SuperCity) category trains, but you can use the “no reservations required” filter in the Interrail planner app to avoid inadvertently boarding this without a reservation.

Or simply don’t use the reservation you have and take a direct train to Prague from Vienna instead, as aforementioned.

Thanks so much. Very much appreciate the input

cheers

jon