It is possible to change it, many members including I have done this successfully.
The onus is completely on the applicant to ensure that changing the NOC (either for a specific job(s) or Primary NOC) doesn't render them ineligible for their chosen program. One must also ensure that the revised NOC is under 0/A/B. If either of these are ignored, there is a possibility that the profile may be deemed ineligible leading to loss of the ITA so it is recommended that one not fiddle with the NOC after getting an ITA unless one understand that making that edit will not have an undesirable outcome.