An REA is usually a prerequisite to a Change Order. Sometimes they work in favor, sometimes they don't. As was stated earlier, it's more informal and allows for negotiation. The REA can be requested at anytime.

I was on a large environmental and demolition project in the Pacific a few years ago. Our contract had us abating about a gazillion square feet of lead based paint on the buildings prior to demo. The government realized that abating all this paint may not be the best way to spend the taxpayers money. To make a long story short, they cut our scope by 75% and wanted to reduce our fee by the same amount. We requested a REA to cover our mobilzation costs, loss of economy of scale, etc.

The REA was requested while we were still working on the island, but the issue wasn't resolved for months after completion.

Clear as mud?