Sen. Bill Cassidy, R-La., who has sponsored a bill to overhaul Obamacare, said Sunday he and his colleagues will introduce a new version of the bill Monday, but didn't elaborate on what the changes would be.

Cassidy suggested a score from the Congressional Budget Office was expected Monday, and said on ABC's "This Week" a new draft of his bill meant "some of what we'll be seeing tomorrow will be no longer relevant."

A CBO score will provide information for lawmakers on whether the legislation would increase the number of uninsured and what the costs will be to the federal government.

Along with Cassidy, the bill is sponsored by GOP Sens. Lindsey Graham of South Carolina, Dean Heller of Nevada and Ron Johnson of Wisconsin. It would transfer Obamacare's revenue to states in the form of block grants and would repeal the individual and employer mandates.

Sen. Susan Collins, who has said it would be difficult to envision her support for a last-ditch Republican effort to repeal Obamacare, said she was concerned about a new bill coming Monday, calling it a "moving target" in which changes were still being made.

"You can't do sound health insurance policy this way," she said on CBS' "Face the Nation." "You need to have extensive hearings. The Democrats must come to the table."

Collins' concerns about the bill, known as Graham-Cassidy, center on whether it provides adequate protections for people with pre-existing illnesses, like cancer, diabetes and arthritis. She also said Sunday she was concerned about changes the bill makes to the Medicaid program.

"It is hard for me to envision getting to yes on this bill because my concerns are so fundamental," she said.

She has instead encouraged her colleagues to go in the direction of fixing Obamacare with Democrats, citing the example of how the Senate Health, Education, Labor and Pensions Committee held bipartisan hearings ealier this month. The top Democrat has urged discussions with Republicans to continue, but it's unclear whether the two parties will be able to arrive at a consensus.

Senate Majority Leader Mitch McConnell has said he intends to bring Graham-Cassidy to the floor for a vote early this week but it faces uncertain passage as three Republican senators have said they oppose the current version of the bill, including Ted Cruz of Texas, Rand Paul of Kentucky and John McCain of Arizona.

Democrats have said they will oppose the bill, which can therefore not pass unless no more than two Republicans oppose it.