Just had a rear wheel replaced by the LBS, basically its a Mavic Kyserium elite which is the same as was on it before, only issue is that its a 2016 model which has replaced my 2015 version.

Slight cosmetic difference but the issue I see is that the 2016 model is 2mm wider, and recommends running 25-32mm tyres. I’ve always run 23mm front and rear and can’t really be bothered buying a new set of tyres.

So will a 23mm tyre work, or will I die? Whats the worst that will happen?

ta

Yes, eventually, blowing off the rim on a 100 kph alpine descent, leading to a faster question 2 answer

23’s fine.

Have you seen the table near the bottom of this page?
http://www.sheldonbrown.com/tire-sizing.html