Scope creep is rarely one dramatic request. It is usually a collection of small additions that feel too minor to price individually, but together consume the margin that made the project worthwhile.
A good change-order process separates the relationship from the calculation. The client can still ask for anything, while the freelancer can show the cost and timeline impact before agreeing to deliver it.
The strongest time to discuss an extra is before the work begins. Clear documentation protects both sides and makes approval easier.