On your mark, get set, go.

This year's Medicare open-enrollment period when the 47 million Americans who use the program can make changes in their 2012 coverage starts this weekend and runs through Dec. 7. Last year, it began on Nov. 15 and ended Dec. 31.

Besides the earlier deadlines, the 2012 plans feature other significant changes, including new enrollment and coverage rules.

"We're seeing these plans getting increasingly complex, with more fine print about what they will pay," says Joshua Greenberg, president of HealthCPA in San Mateo, Calif., which offers fee-based Medicare counseling. "It could have huge financial implications."

For example, if you need the arthritis drug Celebrex and your Medicare plan doesn't cover it, you could be out of pocket $1,176 a year, Mr. Greenberg says. If it is covered, your out-of-pocket cost could be cut in half.

Navigating the process also can be onerous. "To find providers who will accept [Medicare], and to find the right supplemental plan and be confident about your coverage, is overwhelming," says John Cochran, 68 years old, a retired bank CEO who used HealthCPA to help change his Medicare coverage when he started spending more time in Florida than New York and Ohio last year.

Each year, Medicare users have to choose several parts of their coverage. If you use "traditional" Medicare Parts A (mainly hospitalization), B (which covers doctor visits and other outpatient care) and D (a prescription-drug plan) you have to select a specific drug-plan provider. Unless you get supplemental coverage through an employer, you also may want to buy a separate Medigap policy to help with uncovered costs.

Instead of paying for Parts B and D, you could sign up for a Medicare Advantage plan known as Part C which typically works like a managed-care plan with a set group of providers and may or may not also cover drugs. It also could include such extras as dental and vision coverage.

Even if you, or your parents, have been satisfied with Medicare this year, it is becoming more important to review that coverage because many moving parts could trip you up. Plans are even more in flux than normal due to the health-care-law changes in the past year, experts say. The Advantage plan you like might drop doctors or hospitals in its network, for example, or your Part D plan could alter the drugs it covers. (You can find details of your plan, such as the premium, co-payments and benefits, in your "Annual Notice of Change," which you should have received by Sept. 30.)

"It's like investing you make investments, but you reallocate your portfolio periodically," Mr. Greenberg says.

There also might be other options with the same coverage at a lower price. Medicare Advantage premiums are expected to decrease by 4% on average next year from this year, and average drug-plan premiums are estimated to decrease about 2% to $30 a month, according to the federal Centers for Medicare and Medicaid Services, which manages the programs. Remember that these are averages; your plan could increase in price.

Also in 2012, Advantage plans won't be able to charge users for preventive services that are free to users of traditional Medicare, including annual wellness visits, as long as the providers are in the plan's network.

Another change: There is no time limit to switch into a five-star Advantage or prescription-drug plan. (See Medicare's Plan Finder tool at www.medicare.gov/find-a-plan.) Medicare users have one chance to switch to one of these top-rated plans at any point next year, a Medicare spokesman says.

To sort all this out, some Medicare users and their adult children are enlisting professional help. A few such services have a long track record, and others have started up in the past few years. HealthCPA charges $19.95 a month to help people choose Medicare coverage and manage payments throughout the year plus $75 an hour to sort out "thorny" issues, Mr. Greenberg says. Allsup, a Belleville, Ill., firm that advises Medicare recipients and their families, charges $350 to assess all the Medicare options, which includes at least seven to 12 hours of analysis and working one-on-one.

If you change your mind about being in an Advantage plan after Dec. 7, you can "disenroll" between Jan. 1 and Feb. 14, as in past years, and return to traditional Medicare and add a Part D plan, or move into a top-rated Advantage plan. But there is a catch: If you go back to Parts B and D, you may not be able to buy Medigap at that point. The rules and consumer protections vary from state to state, and you may have to pay higher premiums or have some coverage excluded or delayed, according to the Medicare Rights Center, a New York advocacy group.

Free resources include the Medicare Rights Center (www.medicareinteractive.org) and your State Health Insurance Assistance Program, which offers independent counseling (www.medicare.gov/contacts). Be careful: If you seek help from an insurance broker, or through a service run by an insurance company, you probably won't get a review of every option open to you.