Repealing Obamacare won't be that easy. Replacing it will be even tougher.

President-elect Donald Trump has promised to dismantle Obamacare as one of his first acts after taking office.

Yet he can't eliminate Obamacare on day one with the stroke of a pen. Killing the massive program that provides insurance to 20 million Americans would take time to work its way through Congress. Plus, there are many popular provisions that Trump has indicated he would retain in some form, such as covering those with pre-existing conditions.

"I don't think anyone would want to pass a bill overnight that cost 20 million people their health insurance," said Michael Sparer, chair of health policy and management at Columbia University's School of Public Health. "The question is what does he really want to do."

Trump has yet to lay out a detailed plan on how he'd replace Obamacare. But it's unlikely he and Congress will do anything that kills Obamacare for those who sign up for coverage next year, experts say. Open enrollment has already started, and the Obama administration estimated more than 11.4 million people will be insured through the exchanges in 2017.

Any legislation will probably include a delayed shuttering of the exchanges -- and of the federal subsidies program that millions depend on to afford the policies -- to give enrollees time to find coverage elsewhere. The insurance industry will also need a period to adjust since any swift changes to the individual market could throw their businesses into turmoil.

Related: Repealing Obamacare? Here's how Congress could do it

Six years after President Obama's signature health reform law was passed, Obamacare has become an integral part of the nation's health care landscape. Some 20 million people are insured either through the individual health insurance exchanges, the expansion of Medicaid or the ability of children to stay on their parents' plans until age 26. The uninsured rate has plummeted, and stood at 8.9% in the first six months of 2016.

Wiping all this out faces hurdles both politically and procedurally, experts say.

Here's what could happen:

Congressional Republicans have passed dozens of bills to repeal Obamacare. Only one made it to the president's desk, and that legislation provides valuable insight into how Trump and GOP lawmakers may fulfill his campaign pledge, said Timothy Jost, an emeritus professor at Washington and Lee University School of Law.

Passing a bill to overturn Obamacare would require 60 votes in the Senate, and Republicans don't control enough seats to squash a filibuster by the Democrats. But GOP lawmakers, who will have 51 seats in the Senate, could use the budget reconciliation process that only needs approval of a simple majority. This process is limited to provisions that affect federal revenues and spending.

Related: Trump, Pence pitch Obamacare repeal a week from election

Last year, Congress passed a reconciliation bill that would have repealed the mandates that individuals have coverage and that companies with 50 or more employees provide workers with affordable insurance. Also, it would have done away with the federal subsidies by 2018, eliminated funding for Medicaid expansion and canceled a multitude of Obamacare-related taxes. And it would have given Congress two years to come up with a replacement.

Obama vetoed the bill.

With Republicans in control of both Congress and the White House, Trump and lawmakers can try once again to dismantle the core of the health reform law. Still, it could take several months, and going this route would likely prevent Congress from shedding insurance industry reforms, such as requiring carriers to cover those with pre-existing conditions or capping enrollees' annual out-of-pocket spending since they don't involve federal dollars.

Trump doesn't need to pass any laws to severely wound Obamacare, however, Jost pointed out. The President-elect likely already has hurt enrollment on the exchanges for 2017. Those who didn't really want to sign up probably won't and others may shy away thinking the system will be dismantled anyway.

"Will people continue to enroll or will they think things are so uncertain, they won't bother?" Jost said.

Related: Is Obamacare really affordable? Not for the middle class

He could do additional damage by limiting funding to promote sign-ups in the critical final days of open enrollment, which ends Jan 31.

A Trump administration could also stop implementing or enforcing some Obamacare regulations -- such as the restrictions on insurers' offering plans with limited benefits -- and grant waivers to allow states to opt out of parts of the law. And it could broaden the hardship exemption, which would allow more people to avoid the penalty for remaining uninsured.

The incoming administration could also stop defending the federal government in lawsuits, notably the one challenging the subsidies to help reduce low-income Americans' deductibles and out-of-pocket expenses. This would immediately throw insurers into a tailspin since more than half of enrollees receive these subsidies.

But the bigger question is what would replace Obamacare? Neither Trump nor Congress have provided a detailed plan.

Trump has generally said that freeing the insurance industry from Obamacare's grip would lead to more affordable and accessible coverage. His main proposal is to allow insurers to sell policies across state lines. He would promote Health Savings Accounts to let people sock away money tax-free for health care costs, and allow those who buy coverage outside their jobs to deduct the cost of their premiums on their taxes.

Related: Would Clinton or Trump make health care more affordable?

Also, he would turn Medicaid into a block grant program in which states would get a fixed amount of federal funds to cover low-income residents. Proponents say this would give states more flexibility to design programs to cover their residents. But advocates for the poor say block grants would limit funding and therefore reduce access to Medicaid, especially in times of economic stress when enrollment rises.

These measures, however, wouldn't do much to ensure all Americans have access to affordable coverage, experts said.

Among the leading questions is just how Trump would ensure those with pre-existing conditions have access to insurance.

Obamacare banned insurers from turning away those with cancer, diabetes or any other medical issues. But it seems Trump may alter that provision. Vice President-elect Mike Pence said last week that those with pre-existing conditions will not be charged more or denied coverage so long as they have paid their premiums consistently. And Trump's campaign website says he would work with states to establish high-risk pools to cover those with medical conditions. Such state-based pools existed before Obamacare and faced many problems in providing insurance to sick residents.

So just what a replacement plan looks like -- and how long it will take Trump and Congress to come up with one -- remains to be seen.

"Trump says he'll cover everybody," Jost said. "Let's see how he does it."