SAN FRANCISCO/NEW YORK (Reuters) - Volatility could well be in the cards for Wall Street again early this fall, but not for the same reason stocks got rattled in February.

FILE PHOTO: Traders work at the Citadel Securities post on the floor of the New York Stock Exchange (NYSE) in New York City, U.S., July 18, 2016. REUTERS/Brendan McDermid/File Photo

This time the culprit would be the largest-ever shakeup of the stock market’s broad business sectors, which will mean some of the hottest stocks, like Facebook and Google parent Alphabet, will shift from their traditional homes in the top-performing technology sector and into a deepened pool of telecommunications and media stocks.

The sweeping reorganization of the Global Industry Classification Standard, or GICS, means that funds tracking the telecom, tech and consumer discretionary sectors will be forced to trade billions of dollars of stock to realign their holdings by a Sept. 28 effective date.

While the choppiness many investors expect to see is unlikely to hit stocks in quite the same way that wave of the global uncertainty did in early 2018, the fact that so much money must be shifted among index funds in a short time will cause a stir.

In a bid to ensure a smooth transition, leading fund provider Vanguard Group has have already started adjusting its sector exchange-traded funds, or ETFs, while State Street Global Advisors is launching an entirely new fund.

Other investors predict price swings and commotion on trading desks if last-minute sales of Alphabet and Facebook shares by heavyweight technology index funds dwarf demand from a handful of telecom funds buying those stocks.

“There’s probably going to be net selling,” said Andrew Bodner, president of Double Diamond Investment Group in Parsippany, New Jersey. “That will be a temporary scenario, and it could be a good buying opportunity for a lot of those stocks.”

Maintained by S&P Dow Jones Indices and MSCI since 1999 and widely used by portfolio managers, the GICS classifies companies across 11 sectors. The newest, real estate, was split off from financials in 2016. The upcoming changes, which have yet to be finalized, are meant to reflect evolving industries.

Facebook and Alphabet will move from information technology and sit alongside AT&T Inc and Verizon Communications in a broadened telecommunication services sector that will be renamed communications services.

Consumer Discretionary heavyweights Walt Disney, Comcast, Netflix and others will also join the newly defined sector - major changes that will affect investors in sector-focused funds.

Communications services companies will account for one-tenth of the S&P 500, up from under 2 percent for the telecom sector.

To view a graphic on Current S&P 500 sector weights, click: reut.rs/2LkGdax

To view a graphic on S&P 500 sector weights after GICS changes, click: reut.rs/2LkWhJm

U.S. science and technology ETFs have $78 billion in assets, and many will have to sell their shares of Alphabet and Facebook as the changes kick in, according to Thomson Reuters Lipper data. Telecom ETFs, with around $4 billion in assets, will have to buy shares of those companies, while selling some of their investments in AT&T, Verizon and other current constituents to make room for the entrants - trades that will certainly create a surge in volume as well as volatility if not choreographed.

“Stocks will be trading differently than the fundamentals just because of the buying and selling pressure that is going to take place,” said Todd Rosenbluth, director of ETF and Mutual Fund Research at CFRA in New York.

To view a graphic on Technology dominates among U.S. sector ETFs, click: reut.rs/2LgQnZG

The reorganization may also make it difficult to analyze investments as statistics for each sector, like earnings growth and valuation multiples, will change drastically. The telecom sector’s dividend yield of over 5 percent will shrink to 1 percent.

“These changes have really upended the apple cart, and investors need to ensure they know all of the changes before they take effect,” said Matthew Bartolini, head of SPDR Americas research at State Street Global Advisors, of the upcoming restructure.

Wealth management firm Exchange Capital Management is reviewing client accounts that include sector ETFs to identify which will be most affected by the changes, said portfolio manager Andrew Stewart. The Ann Arbor, Michigan firm will weigh the benefits of rebalancing clients’ ETFs against the taxes that would have to be paid on capital gains resulting from their sale.

“The powers at GICS have made a very rational, academic decision to reclassify these sectors,” Stewart said. “But I don’t think their systems are built to pay attention to the needs of Jane Doe’s retirement plan.”

To view a graphic on Current S&P 500 Telecom Sector, click: reut.rs/2Jsmpoo

To view a graphic on S&P 500 communications sector components after GICS changes, click: reut.rs/2JsqF7m

To avoid having to make large trades when the changes go into effect, Vanguard has pegged its technology, telecommunications and consumer discretionary sector ETFs to temporary benchmarks adjusting gradually over four months. Its recently relabeled Communications Services fund already includes tiny investments in Alphabet and Facebook.

Bank of America warned in a recent report that the changes will leave the new communications services sector more overbought than any other due to its high concentration of popular stocks Alphabet, Netflix, Facebook.

Ivan Cajic, head of index research at ITG in New York, said he expects most passively managed funds to wait until the changes go into effect to realign their portfolios in order to remain true to the indexes they track.