Disagree: With written concurance of the Client, Engineer-of-Record and Mfr, it can be sone 'legally' for most specifications. Reason being that the specification is being required by the customer, and if the specification allows it, it can be done.
For things built the ASME Code, the answer is NO. Reason being, that a 3rd-party approved QC Manual is mandatory. You would have to properly revise the QC manual and then reobtain that 3rd-party approval.
In a well written and thought out QC plan somewhere on page 1 there should be a prominent sentence along the lines of "No deviations from the procedures set forth herein are allowed."
However, on pages 173 - 186 there should be sufficient flexibility written in to allow for cases that weren't fully considered when the plan was written.
So in reality you should never need to deviate from the plan because the plan allows just about anything, as long as it is documented an controlled.
Of course, even if that flexibility from pages 173-186 is NOT built in, there's always the NCR route. Deviate, write the non-conformance, disposition to leave it alone with acceptance by the client.
Any QC program is only as good as the people who enforce it. I've seen 1 page QC documents which were created just for the sake of having one to fulfill a subcontractor requirement, and others in the nuclear arena that practically need a supercomputer to maintain and are enforced to the letter.
The answer to the original question posed by the OP, in my opinion, is impossible to justify without any context whatsoever, unless of course answered with an "it depends."
