I'd take some more time to change.

Edit: what station are you coming from?

I am coming from Ljubljana, Slovenia. Right now I dont seem to have any other options, but since I am not travelling until the end of July perhaps there will be some timetable changes in the meantime?

You can change on the EN to Warsaw also at Graz HBF, where the train starts from 12th June. If your train has not to many delay, the train will wait. Please at Spielberg (border station Slovenia-Austria) go to the Austrian ÖBB train conductor and say that you have to get the EN to Warsaw. The train could start in Graz and wait for you in Vienna. 

Or more easy use a train before. 

In Graz, EC 150 from Ljubljana is still behind the EN. Only in Wiener Neustadt or Wien Hbf (8 min.) you can make the connection.

According to this post, the night train will wait in Wien Hbf in case of a delay (the poster works for ÖBB).

@rvdborgt oh, ok Thank you. I didn’t notice that the EN runs before the EC and it changes before Wiener Neustadt. Good to know. 

Thanks so much for the information! Very useful!