By September, all nonstop Delta Connection flights to airports farther than 750 miles from Salt Lake City will have first-class cabins.

The upgrade is part of a $1 billion investment Delta Air Lines is making to improve products and services across its network of domestic and international routes, spokesman Trebor Banstetter said Tuesday.

Last week, the airline said it would retrofit its fleet of single-class regional aircraft to add first-class service on all U.S. flights of at least 750 miles or that take more than 2½ hours of flight time.

When the upgrade is finished, 50 domestic routes providing Delta Connection flights with only economy-class service will have two-cabin service, with first and economy-class seats, Banstetter said.

By fall, Delta's fleet will include 219 regional aircraft with two-class service flying on 272 routes.

The converted aircraft will give members of Delta's frequent-flier program more opportunities to upgrade from economy to first-class.

Six routes start from Delta's hub at Salt Lake City International Airport. The flights are to Austin, Texas (1,085 miles); Tulsa, Okla. (926 miles); Kansas City, Mo., (918 miles); Oklahoma City (864 miles); Omaha,Neb. (839 miles); and Vancouver, B.C., in Canada (798 miles).

Those routes are flown mainly by St. George-based SkyWest Airlines, which uses Bombardier CRJ700 regional jets. SkyWest shares the Austin and Kansas City routes with another Delta Connection carrier, Mesaba Airlines, based in Eagan, Minn.

"We don't have exact dates" for when SkyWest's jets will be ready, Mike Kraupp, vice president of finance for the St. George-based carrier, said. "All we know is, by September."

The Bombardiers will be equipped with nine or 12 first-class seats and 56 or 64 economy-class seats, depending on how they are configured, Banstetter said.

"A lot of our customers appreciate the opportunity to upgrade (from economy to first class)," he said. "That's something they value, particularly for a flight that tends to get a little long."