Boeing engineers have been in Japan installing reinforced lithium ion batteries on five grounded 787 jets owned by All Nippon Airways.
It means the Dreamliners could be ready to return to the air in about a week.
ANA would then make test flights before putting them back in service in June.
The 787s have been banned from flying by regulators since mid-January after batteries on two of them overheated.
ANA has not said how much the 787’s grounding has cost it to date, though it has said it lost about 690,000 euros in revenue per plane in the last two weeks of January.
The grounding has cost Boeing an estimated 460 million euros.
ANA is the world’s biggest operator of the carbon-composite aircraft with 17 of the planes. After ANA, the biggest 787 operator is local rival Japan Airlines with seven jets, followed by United Airlines and Air India with six each.
Investigators in the United States and Japan have yet to discover exactly what caused a 787 battery onboard an ANA jet in Japan and one on another JAL Dreamliner parked at Boston’s Logan Airport to overheat.
Europe follows US
European safety authorities are backing a decision by US regulators to allow the planes to resume flights after a three-month grounding.
The Federal Aviation Administration (FAA) in the US on Friday approved a Boeing plan to encase the 787s lithium ion batteries in steel box, install new battery charges, and add a duct to vent gases directly outside the aircraft that could cause overheating.
The FAA is expected to issue imminently an “airworthiness directive” formally lifting the flight ban, in place since January.
When it does so, the European Aviation Safety Agency plans to adopt the same measure, a spokesman for the regulator said.