My MW and Consultant MW told me when I was booking my ELCS that it was extremely rare for labour to progress so quickly, that a planned ELCS couldn't be carried out.
Of course it can happen, for sure. Like pretty much anything else under the sun during labour and birth.
I was told that if I had any feeling at all that labour was starting, to get into the hospital asap, where they would assess me and proceed with the ELCS accordingly.
As it turned out, my waters went early, before my planned ELCS date, so this was all good and necessary advice!
And it was totally fine. I went in, was assessed, and they simply did the CS within 24 hours. No problem. As we know, few babies arrive on their due date, however their mothers give birth, and hospitals these days schedule CS's as close to 39 weeks as possible, to make sure the baby is full term and to minimise any problems caused by early delivery.
So it's a scenario they are very familiar with (labour starting before ELCS date), and can brief accordingly.