By Tami Luhby

NEW YORK (CNNMoney) -- The Trump administration is taking its first steps to overhaul Obamacare.

It issued a proposed rule Wednesday aimed mainly at quelling insurers' concerns and stabilizing the marketplace for 2018.

While President Trump wants Congress to repeal the Affordable Care Act as quickly as possible, his hands are tied by two important factors: He has promised to have an orderly transition out of Obamacare, so people don't lose their coverage, and he must defer to Congress to actually change the law.

However, his administration can adjust regulations even before lawmakers act.

The rule change would tighten the provisions to apply for coverage outside of open enrollment and require consumers to pay any back premiums due before signing up again. And it would cut the open enrollment period for 2018 in half, so that Americans could only sign up between November 1 and December 15.

Insurers would gain greater flexibility in how much their policies would cover in each tier -- for instance, Silver plans currently must cover 70% of out-of-pocket costs, on average.

The changes would also eliminate the federal government's review of whether insurers have enough doctors in their networks. And it would give insurers more time to have their 2018 plan reviewed and finalized by state and federal regulators.

"This proposal will take steps to stabilize the marketplace, provide more flexibility to states and insurers, and give patients access to more coverage options," said Patrick Conway, acting administrator of the Centers for Medicare & Medicaid Services, which oversees Obamacare.

The rule could take affect as soon as next month. The agency is requesting comments until March 7 and will then likely act after that.

The rule does not contain one of the more controversial changes insurers want -- allowing them to charge older enrollees more. Under Obamacare, carriers can only charge older policyholders up to three times what they charge younger ones. Insurers are looking to widen that band to five-to-one. This provision is in the law and can only be changed by Congress.

Health insurers, who were already pulling out of the exchanges, are growing more jittery about participating for 2018. Humana announced Tuesday it was pulling out of the individual market completely. Carriers want more certainty from Congress about what changes will be made and when, however Republican lawmakers remain divided.

"Today's proposed rule is an effort to stabilize the exchange markets until an ACA replacement plan is approved and implemented," said Caroline Pearson, senior vice president at Avalere Health, a consulting firm.

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