Moody's Investors Service downgraded bond ratings for Dallas-Fort Worth Airport on Friday, saying it expects the airport's debt to increase as it continues to issue bonds to pay for its $1.9 billion terminal renovation program."Total debt outstanding, already among the highest in the nation for airports, is expected to become the highest during 2013," Moody's analysts wrote. "This will bring debt per enplanement to over $200 and debt per O&D enplanement over $400 for an extended period of time."DFW Airport is in the early stages of its terminal renovation program, in which it will overhaul all of its older terminals. Work is nearing completion the first phase of Terminal A, about a third of the terminal, which is expected to reopen soon. The work was supposed to have been completed late last year but has taken longer than expected to complete.It's expected to take three years to complete renovations of each of Terminals A, B, C and E.Other factors raising the risk for DFW investors, Moody's said, include the ongoing bankruptcy of its main tenant, American Airlines, and increasing competition on the horizon from Dallas Love Field when the Wright Amendment restrictions on Love Field flights are fully removed late in 2014.Still, Moody's issued a "stable" outlook for DFW bonds based on "expectations that the airport's operational, geographic and financial strengths as well as the economic strength of the Dallas-Fort Worth metropolitan area will allow DFW to successfully navigate a period of heightened risks and high leverage."It noted the airport's "large, affluent service area" and "strong and experienced management team" as strengths and said that "a successful merger between American Airlines and US Airways would make the airport the largest hub in the network of the world's largest carrier."