Did this recently

it is simple procedure
all insurance company have the standardize procedure

it is basically informing your old car insurance that you want to transfer your NCD
they will calculate the left over months of your insurance, these left over (let say 6 months more) will be recalculate to be 0% NCD
so you just top up the cost, usually continuing the old insurance company

then the company will do the procedure, informing you the NCD is no longer linked to the old car
you just tell the new car SA your NCD can be transfered already
they will do the process

as the SA able to check your profile to see your NCD is not linked to any car
and the will just registered the new car to your profile

ask your SA and your old car insurance what can be provided to confirm the NCD transfered out already
some said official letter, but in this days, eletronically, all data is pool to one source

once old insurance done with the process
new car can get your data and registered the new car