What is the best way to handle changes to product requirements or scope during or after User Acceptance Testing (UAT)? How dangerous is it to make those changes, and how can the danger be mitigated?
The scope can be changed at anytime, provided those changing accept the required changes to budget and schedule. You can't stop people from continually trying to change the scope until they realize it costs money, providing a few estimates of the cost for requested changes from higher-ups will help them make that connection. Doing this is unlikely to stop them from requesting changes, but it is likely to stop them from complaining when you deny those changes, and will help you identify the changes you need to make when they understand the cost of that change.
I think the best way to deal with these change requests is to present a revised schedule including the change. And if it's after UAT, then the new schedule includes a second run through UAT.
When they go 'but we don't need that' you can ask why not? Doesn't the product need to be accepted by the user? If they haven't seen it, they can't accept it, right?