Workers at Kennedy Space Center in Florida are reinforcing 32 structural support beams, called stringers, on the tank's midsection, Florida Today reported Friday.
An additional 63 stringers may get similar support, but shuttle program managers say they won't make that decision until engineers confirm the changes won't produce any unintended consequences.
NASA says the shuttle won't be ready in time for the next available launch window of Feb. 3-10, and the next opportunity would be Feb. 27 to March 6.
"That first window in February is no longer available based on the amount of work that's left to be done and testing they would like to complete," said Kyle Herring, a shuttle program spokesman at Johnson Space Center in Houston.
Discovery's 11-day mission to deliver equipment to the International Space Station is one of two or three planned this year before the shuttle fleet is retired.
Endeavour is scheduled to fly April 1, and if additional funding is found, Atlantis would fly the final mission, possibly in late June.