Start with a simple qualitative assessment. Rate the impact of the requested change on each of the nine PMBOK knowledge areas, using a standard Likert Scale where 5 is unacceptable impact and 1 is no impact. Engage the appropriate people in the discussion. For example if you have a Contracts Officer, you would certainly want their opinion on potential impacts to Procurement, or existing contracts.
Deeper analysis can be done of course, but I've found that this simple approach provides a surprisingly good picture of impacts, typically sufficient to make a decision.