Like an old beater that needs constant repairs, target-date funds are heading back to the shop for a tune-up. This time, it's to tweak exposure to foreign stocks in these all-in-one portfolios that are meant to gradually get more conservative as an investor approaches retirement.

This fall Vanguard which together with Fidelity and T. Rowe Price controls more than three-quarters of the about $300 billion in target-date assets announced that it was effectively increasing the percentage of foreign stock in its target-date funds from 20% to 30%. This is similar to Fidelity's move to having 30% foreign exposure in each of its Freedom fund portfolios over the past year. Both companies say the change is not a response to recent market events but rather an attempt to diversify the funds without increasing risk. Still, it raises the question, "Were you wrong before, or are you wrong now?" says Mercer Bullard, president of the investor-advocacy group Fund Democracy.

Target-date fund investors should be used to having their investments remodeled. Following the dismal performance of target-date funds as a group during the 2008 crash, fund companies brought out the toolbox, in some cases reducing the funds' allocation to equities or adding less-risky assets like Treasury inflation-protected securities. This was all in an effort "to make sure target-date funds have strong returns with lower volatility," says Laura Lutton, editorial director at Morningstar's fund-research group. And while global diversification is generally a good thing for investors (Fidelity's Freedom funds are, for example, up an average of 10% over the past year), some critics caution against dramatic changes, especially since a weak dollar has created a tailwind behind foreign investments.

"When the headwind created by a stronger dollar begins to blow, people could look at this 30 percent exposure and say, 'Wait a minute, maybe we've overdone it here,'" says Dan Wiener, editor of the newsletter Independent Advisor for Vanguard Investors. For now, investors can track their fund's changes by looking at its prospectus or their fund company's web site.