Sachin,
You can find the "best" answer in the PMI Code of Ethics and Professional Conduct. Here is what chapter 2.2.2 says:
2.2.2 We accept only those assignments that are consistent with our background, experience, skills, and qualifications.
Based on just this statement alone, the correct answer would be: "You are NEVER allowed to take an assignment that is beyond your expertise and know how." But of course, that would be silly because it means you can never learn anything new or grow into a new area. That is why you also find the following comment just below 2.2.2 in the Code of Ethics:
Comment: Where developmental or stretch assignments are being considered, we ensure that key stakeholders receive timely and complete information regarding the gaps in our qualifications so that they may make informed decisions regarding our suitability for a particular assignment.
So in other words, the "best" answer to the question is this: You may accept the new position, but you have to inform the hiring manager about your qualification gaps.
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC