KKR & Co.'s Energy Future Holdings Corp., struggling to avoid default, is enjoying a $450 million windfall at the expense of bondholders.

Energy Future, formerly called TXU Corp. and taken private by KKR, TPG Capital and Goldman Sachs Capital Partners five years ago in the largest leveraged buyout, exchanged $1.15 billion of new notes last week for old ones with a face value of $1.6 billion. The move came after the old securities tumbled when the firm said in an Oct. 30 regulatory filing that it may be liable for $23 billion of taxable income if it cuts ties to units that default.

The disclosure “allowed them to get a better” deal on the exchange, Andy DeVries, an analyst at independent bond research firm CreditSights Inc., said in a telephone interview. The filing probably was intended to rattle bondholders and cut the value of the outstanding debt before the swap, he said.