Kulya Pakakrong, the acting president of Airports of Thailand, said: "Moving some passengers to Don Muang is better than putting up an immediate investment of 47 billion baht for the new phase of expansion."
He was referring to a new cost of almost $1.5bn for Suvarnabhumi.
"We will take six months for the study, which will recommend how we want to utilise Don Muang, for instance requiring all the low-cost airlines to come here or making it for regional flights," he said.
Suvarnabhumi, which means "Golden Land" in Thai, is Southeast Asia's largest airport, and Thailand had hoped it would emerge as a regional rival to Hong Kong and Singapore.
But the military-appointed government decided in February to re-use Don Muang after repair work on Suvarnabhumi's runways and taxi-way caused congestion and disrupted flights.
Only national carrier Thai Airways, its low-cost arm Nok Air and another budget airline, One-Two-Go, have so far decided to return to Don Muang. Others said they preferred to operate from the new airport because it handled both international and domestic flights.
Many passengers were pleased to return to the old, smaller airport of Don Muang, which is now expected to handle 140 flights and an estimated 18,000 passengers a day.
"I like this one, the new one is so big and you have to walk very far," Dutch tourist Kelly Vandersteeg told Reuters after returning from the northern province of Phitsanulok as workers finished painting the wall of the terminal.
But Hideaki Komiya of Japan was not happy. He and six other family members had to take a 60km taxi ride to Don Muang after landing at Suvarnabhumi to connect to a flight to the resort island of Phuket.
"It is very far from the new airport to the old airport," said Komiya.