American Airlines will combine the frequent-flier program of merger partner US Airways into its AAdvantage program, starting Saturday.

The move to a single frequent-flier plan will take several days and will mesh 30 million US Airways' Dividend Miles members with 70 million American customers, American said in an e-mail message to travelers Tuesday.

Although the two carriers officially merged in December 2013, the process of combining frequent-flier programs, obtaining a single operating certificate from the Federal Aviation Administration, and getting to a single computer reservation system are major steps that will not be completed until late this year.

Tuesday's e-mail assured US Airways customers they will not lose any of their miles when the programs are combined. "Once we're finished, we'll send you an e-mail with your new account number," American said.

If they have not gone online to merge their Dividend Miles and AAdvantage accounts, those with frequent-flier accounts at both carriers will get a new AAdvantage number. In April, an online link will be available to help travelers who end up with two AAdvantage accounts combine them, American said.

Dividend Miles reward bookings and mileage-upgrade requests will be disabled at midnight Thursday for the transfer, American said. "Once your miles have been transferred to your AAdvantage account, you'll be able to redeem on aa.com and with AAdvantage reservations," the statement said.

For US Airways' most frequent fliers in the chairman's preferred tier, the last day to redeem current upgrade certificates is midnight Friday, American said. "As a new AAdvantage executive platinum member, you'll receive eight systemwide upgrades to use on both airlines through Feb. 2016," the e-mail said.

Dividend Miles members who fly occasionally "will see quite an improvement, such as one-way awards at half the price," said Brian Kelly, founder of ThePointsGuy.com, a blog and website for maximizing airline frequent-flier miles and credit-card points.

US Airways allows frequent fliers to redeem travel rewards only for round-trip flights, whereas American permits the miles to be used on one-way trips.

American permits "off-peak" travel miles for European travel between Oct. 15 and April 15. US Airways permitted the travel only between Jan. 15 and Feb. 28.

"It's an amazing deal, so that's an improvement," Kelly said.

American fliers also can book off-peak travel benefits on partner airlines, such as British Airways. US Airways required that rewards be used only on its flights.

The biggest change for US Airways' most frequent jet-setters, who log 25,000 miles or more a year, will be the end of automatic complimentary upgrades for US Airways silver, gold, and platinum customers.

Only the top-tier fliers - US Airways chairman's preferred and American's executive platinum, who fly at least 100,000 miles a year - will continue to get unlimited upgrades from coach to first class on domestic flights.

Lower-level elite fliers in US Airways' silver, gold, and platinum levels will have to earn, or pay for, upgrades on flights longer than 500 miles.

On trips shorter than 500 miles, all elite levels will still get free upgrades on North American flights. US Airways' elite-level fliers were accustomed to free upgrades on domestic flights of all distances.

The combined American-US Airways is generally adapting the American AAdvantage program, with three levels of elite-status membership: executive platinum (100,000 miles flown annually), platinum (50,000 miles), and gold (25,000 miles).

US Airways has a fourth level, platinum preferred, for customers who fly 75,000 to 100,000 miles a year. That tier will disappear, and those fliers will be mixed in with American's platinum level and US Airways' current gold members, who fly just 50,000 miles a year.

For now, American said, it will continue to reward miles for travel based on distance flown, not fares paid. That benefits leisure travelers, who hunt for bargain fares, in contrast to business travelers, who often book expensive last-minute tickets. Delta, United, Southwest, and JetBlue airlines have adopted revenue-based travel rewards programs that favor passengers who spend the most.

US Airways and American transport 77 percent of air travelers in Philadelphia.

lloyd@phillynews.com 215-854-2831