The key thing to note is not to accept changes until you have looked at all your options - you only have one chance to amend (or refund). Look at the Disruption thread and EU/UK261 thread to understand your rights. Sorry it seems pedantic on language, but UK261 rights depend if a flight has actually been cancelled (different flight number) or the timing has been changed.

I find Vienna a very pleasant city to visit, lots of history, culture, museums and the opera. There are also fun pubs on the river and wine growing areas just outside the city, and mountain areas in easy reach by public transport.