It's a little strange that they're changing the interview format on such short notice, but my guess is that it has to do with interviewer availability and/or maybe a need to make a decision in a certain time frame. In any case, I don't think this has anything to do with you as a candidate.
If you've been told that your interview will be focused on casing, I would recommend doing some live case practice! How best to do this varies for each candidate, so it's important to go for a prep approach that really fits your needs and preparedness level. In general, my two cents would be: I don't know how comfortable you feel with cases at this stage (I assume fairly comfortable since it's the final round?), but I would go in and do a couple of live cases in the first few days, figure out what your main areas of improvement are, spend some time exclusively focusing on those, and then do a few more live cases. I would also really recommend taking some time to relax and do something fun before the interview, to ensure you are well-rested and able to perform your best. Good luck, and feel free to get in touch if you have any more questions!