Rybak's lawyer, John Corrigan, confirmed the plea change Friday to Channel 3's Tom Beres.
He said Rybak is "at peace with his family and at peace with his maker."
Corrigan said there will be more to say "at the right place and the right time."
Rybak, 53, of Independence, is the business manager of Plumbers Local 55 union. He is expected to admit to bribing Dimora in various ways in exchange for a pay raise for his wife who works for the county.
Rybak is charged with bribery, embezzling from the union and conspiring to obstruct the corruption probe.
Dimora allegedly received free plumbing work at his Independence home and an ice maker.
Federal prosecutors released quotes from a taped conversation between Dimora and Rybak. They discussed the possibility of arranging sexual favors for Dimora.
Rybak referred to former New York Governor Elliot Spitzer, who resigned as governor after admitting to patronizing a prostitute.
"Remember Spitzer, if I go down, you're going with me," Rybak said to Dimora on the taped conversation.
"We're all going down together, baby," Dimora replied, in the taped conversation.