Sprint Corp., Overland Park, Kan., lowered the long-term rate of return assumption for its $2.4 billion defined benefit plan to 9%, from 9.5%, according to the companys annual report. Sprint also lowered the plans discount rate assumption to 6.75%, from 7.5%. A company spokesman said Sprint will likely be required to make a contribution to the plan this year. The plan, which was underfunded by $9 million in 2001, was underfunded by $1.1 billion at the end of 2002, according to the report. As a result of the additional liability, the company took an after-tax charge of $713 million in December.