The reconstruction of Oroville Dam’s flood control spillway is likely to cost as much as $500 million, state officials said Thursday, as design changes and unexpected additional work has inflated the cost of the project. Originally budgeted at $275 million, the repair has grown to cover a greater level of protection for the dam’s emergency spillway – whose near failure in February sparked the evacuation of 188,000 downstream residents – as well as unforeseen problems in the bedrock beneath the main spillway, said Erin Mellon, a spokeswoman for the state Department of Water Resources.