It would depend on quite a few variables. You will need to check the contract for your present job, to see what notice to leave you must give the employer, but then, s/he may require you to work out the full time, or not. You need to be sure that the new employer is happy to give you a start-date after that. You need to be absolutely certain that the new job meets all the requirements for VOC, or find out if you will have to get a new visa.
Contact INZ with all the details. If everything is in order for you to make a change, whether by VOC or a new visa, they are very good about arranging the date of change so that you are correctly covered for completing your time in your old job, then starting in the new.