From the IRS website:

"These refunds are expected to begin in May and continue into the summer.

The agency will do these recalculations in two phases.

First, taxpayers who are eligible to exclude up to $10,200.

Second, those married filing jointly who are eligible to exclude up to $20,400, and others with more complex returns."

Nothing specific on what they mean by "more complex returns," but probably those where a change in AGI means a change in credits or deductions. 

And of course, no word on whether they will pay interest without a claim having been filed.