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College savings plans are a great way to save for education. But not all college savings plans are great.

Most state-sponsored 529 college savings plans, which allow you to invest in a tax-advantaged account for future education costs, have improved significantly in recent years, says Madeline Hume, analyst for multi-asset and alternative strategies at investment research firm Morningstar. Plans have lowered fees, improved investment options and smoothed investment "glide paths" to reduce risk.

But not every plan is keeping up. Morningstar recently downgraded eight state plans and advised most savers to avoid five others, often for excessive costs.

If you are saving for a child's education in a 529 plan, or want to start, it's a good time to review your options because there may now be a better choice.

Aiming for a smoother landing

Most of the money saved in 529 plans is invested in age-weighted options that reduce exposure to stocks as the child gets closer to needing the money. In the past, 529 plans might keep the same portfolio of investments for four years or more before selling and moving into a supposedly less risky portfolio in a single day, Hume said. But those sudden movements weren't risk-free.

"Especially if there's a large market drop on a particular day, that investor could lock in losses that may be hard to recoup," Hume said.

Today, many plans mimic target-date retirement funds, which reduce risk gradually. Even plans that still sell one portfolio of investments to buy another tend to do so more often to reduce the possibility of locking in big losses and give investors a smoother ride, she said.

California's decision to move its ScholarShare College Savings plan to a progressive glide path helped earn it a gold rating this year, up from last year's silver. Three other plans — Bright Start College Savings in Illinois, Invest529 in Virginia and my529 in Utah — also earned top marks for their glide paths, low fees and best-in-class investment options.

Most plans continue to slash fees

The investment industry has been slashing costs and eliminating commissions at a "dizzying" pace, so plans that haven't done so have started to look unattractive, Hume said.

Morningstar downgraded Nevada's the Vanguard 529 College Savings Plan, a top-rated plan since 2012, from gold to silver status for this reason. Its fees remain below average but are no longer among the cheapest, Hume said.

Cost was also the reason that four other plans received negative ratings. Those plans include North Dakota's College SAVE Plan, New Jersey's Franklin Templeton 529 College Savings, Arkansas's GIFT College Investing Plan and Nebraska's TD Ameritrade 529 College Savings Plan.

The fifth plan to flunk out was Nevada's USAA College Savings Plan. Morningstar downgraded the plan after Victory Capital Holdings bought USAA's asset management business and added its own managers to all the underlying equity funds. The change happened before Nevada state officials had time to vet the changes, Hume said. Strong state oversight is a key factor in Morningstar's rating system because it deters investment firms from making money at the expense of investors.

What you should do now

You generally can change 529 providers once every 12 months without triggering IRS taxes and penalties. But you will want to consider state tax treatment, as well.

Most states offer residents a tax break for 529 contributions and may require you to pay that back if you transfer the account to another state's plan. If you get a tax break and your plan isn't on Morningstar's naughty list, it may make sense to stay put depending on the size of that break, the state's policies on paying it back if you move and the plan's quality. Check the plan's site for details.

If your state plan did get a negative rating, you have alternatives. Many states offer more than one plan, and Nebraska, New Jersey and Nevada all have better-rated options. Also, Arkansas is one of the seven states that give a tax break for investing in any state's plan, not just its own. (Arizona, Kansas, Minnesota, Missouri, Montana and Pennsylvania are other "tax parity" states.) Plus, your state could clean up its act.

Not all states offer tax breaks, of course. California, Delaware, Hawaii, Kentucky, Maine, New Jersey and North Carolina don't offer tax deductions or credits for 529 contributions.

If your state doesn't reward you for staying or punish you for straying, there's little downside to moving your money to a better plan.

Liz Weston is a columnist at NerdWallet, a certified financial planner and author of "Your Credit Score." E-mail: Twitter: @lizweston.