Normally change request are handled by preparing an Impact analysis document and then doing re-estimation. Example you have an on going project, which has a customer table. Now customer want to also have addresses assigned to it. So you normally raise a change request and then do an impact analysis of the same. Depending on the impact you estimate and let know the client about the financial aspect of the project. Once client sign off or the upper management agrees to the change request you move ahead with implementation.