SO. MUCH. PANIC. Unfortunately these things do happen and usually at the last minute too.
This happened to my cousin a few years ago, the reception venue cancelled last minute and she had to find another venue to host 300+ guests within a small timeframe before the wedding. After a lot of panicking and stress she found another venue and we created a notice card and sent it out to all the guests. The good news is everyone got the memo and turned up at the right place - mission accomplished.
From my experience the best way to tackle this is to:
1) Send a physical notice and;
2) Send a digital notice via messaging and/or email and ask people to reply as acknowledgement.
You can even take it one step further by creating a Facebook event with the changes and ask people to RSVP on that.
Touch wood that this happens to you, but for clients of The Hello Bureau I am more than happy to design a complimentary notice card for you (printing is separate but I can assist you with that if required.) Even if it has been months since your invitations went out, just contact me and we'll get the notices out ASAP!