I asked this very question (and as it turned out, my waters went 10 days before my ECLS date, so I'm glad I did) -
They simply do it earlier, provided you are full term and all is well otherwise. My MW said the key thing is to go in as SOON as you think you might be in labour, which is the opposite advice given to women aiming for a VB, who are told to stay at home to start with. That's in case of the (very small) chance that labour progresses super fast, and it gets too late to do a CS.
You will be assessed as to how soon they need to get you into theatre, and you just go in as soon as they get a free slot. My waters went at about 8pm at night - I went in to be assessed at about 10pm, and the monitor showed I was contracting but I couldn't feel it. They gave me the option of staying overnight and or going home and coming in at 8 the following morning. I did the latter, to get a night in my own bed. I went in at 8 am, and was eventually in theatre at around 4 pm, I think. It would have been sooner but theatre was busy with EMCS's and instrumental deliveries, which rightly had priority.
It depends a bit on your situation - if you really want your CS or it is medically vital, do the above. If you are able and want to go for a VB, then you can go for that.