Airlines to Boost Fares in Hundreds of Markets : Travel: The changes involve elimination of temporary discounts on tickets popular with business travelers.

Air fares are set to go up as much as 40% in hundreds of markets Thursday, as temporary discounts on fares most popular with business travelers expire.

In some cases, the unrestricted coach fares will rise above the pre-discount levels as airlines tack on premiums for nonstop service in their dominant markets.

Discount fares are not disappearing entirely, however. On dozens of highly competitive routes, such as Los Angeles-Chicago, the discount on unrestricted coach travel will remain in effect.

Travel agents advise consumers worried about fare hikes to buy their tickets today, since airlines will allow flight changes later for a $25 service charge. With round-trip, 14-day-advance-purchase coach fares between Los Angeles and New York rising by $82, to $400, on Thursday, people would realize savings even if they had to change their plans later and pay the service charge, said Thomas Nulty, president of Associated Travel Management in Santa Ana.

The latest round of changes further confuses the industry’s month-old attempt to simplify air fares. “The structure is gradually reverting to more complexity,” said pricing analyst Donald S. Garvett of SH&E;, a New York-based airline consulting firm.

Led by American Airlines, the industry on April 9 moved to eliminate discounts while lowering everyday unrestricted fares by at least 38%. At American, the pricing overhaul eliminated all but 70,000 of the 500,000 fares in its reservation system.

Trans World Airlines, in keeping with its strategy, immediately lowered its business-class fares below the industry’s new price schedule. The move forced American and the rest of the industry to match TWA’s discounts in 1,500 well-traveled markets.

TWA’s discount fares are also set to expire Thursday. Industry analysts said TWA might extend the discounts, a step competitors might again feel compelled to match.

Chairman Carl A. Icahn previously said that TWA, which is operating under bankruptcy court protection, plans to underprice competitors. However, a TWA spokeswoman could not comment Tuesday on the airline’s plans.

Meanwhile, Delta, Northwest and USAir said they are charging premiums in markets they dominate. A sampling of fares shows that the premiums can range from $20 to $80 over the next highest fare in the market.

A look at Los Angeles-Atlanta fares shows how prices are changing--and suggests that consumers can benefit by price shopping. The current one-way unrestricted coach fare on United, Northwest and American is $275. Delta, which alone offers nonstop service to its Atlanta hub, charges $305.

On Thursday, United and American plan to raise their one-way coach fares to $300, while Northwest will boost its fare to $380. Delta plans to charge $410.