when u purchase your new car, let your agent knows that u want to transfer ownership of current car to new car.
for a smooth process, u need to do the following:

1) purchase new car insurance will full (no NCD).
2) ask your agent to trigger the transfer of NCD
3) old insurance company will recalculate & ask u to pay the additional amount of no NCD of remaining insurance tenure.
4) new insurance company will contact u & issue the refund of your new car NCD. they will usually tell u takes 2 weeks for refund, but most of the time u'll get it within days.

of coz, if your new car is using the same insurance company as old car, the process will be much seemlees.
additional fact: 1 person can hold 2 active NCD.

depends how many % of NCD u got.
when u hold active NCD, each year u'll get increase discount until 55%.
but if u let your NCD idle, each year will decrease discount.

so if u only hav 2 years worth of NCD discount for example, after 2 years of idle, already bck to 0%, so might as well as stick wit your current NCD.