Is it necessary to change a speedometer in miles to kms to register a UK car in France?

no you do not have to change this.

GL

Your speedo normally has KPH on it as well as MPH,so no need to change it.You will have to constantly remember that you should be looking at the smaller KPH display and be very careful regarding speed limits.You soon get used to it. :-)MM

you can make a conversion half circle or bar yourself to help you stay in the speed limits

no.