They're learning to canoe now
The turning point came in 2005, when Lynn turned 12 and was referred to the National University Hospital.
Suspecting that she could be suffering from AADC, paediatric neurologist Stacey Tay performed a spinal tap and sent the fluid to the US for testing.
"This is a very complex test performed in only several accredited clinical labs in the world," Prof Tay said, adding that some mild cases may be treatable and better mobility can therefore be achieved earlier.
Within three to six months of diagnosis and treatment, the Chang sisters were "walking better and did not need their wheelchairs", Mrs Chang recounted.
"According to Prof Tay, because of the rare nature of the condition, the amount of medication administered was on a trial-and-error basis. Sometimes more, sometimes less, and it took a few months to see how they responded," Mrs Chang added.
Joyce even managed to move from a special school to a mainstream one.
Around the same time, Mrs Chang also read in a magazine article that therapy in China could also help.
Not wanting to leave any avenues unexplored, she got in touch with the Chinese physicians concerned and took her daughers to China, doing so with the blessings of her husband and Prof Tay.
Mrs Chang said: "It was the year that Lynn was sitting for her PSLE and we applied to have her exempted. We were there for 2½ months. The treatment was intensive and painful, especially for Joyce. But I'm glad that I took that chance. Look at my girls now."
For Joyce, who has a club foot (a congenital foot deformity), which seemed to be rotated internally at the ankle, walking was near impossible.
"But the intensive and painful massage therapy in China helped. She was able to walk after we took her there," Mrs Chang recounted.
The girls can now go through a day without having to nap and are even learning to canoe.
Today, Lynn is 22 and studying at Republic Polytechnic and Joyce, 19, is finishing her secondary school education at Unity Secondary.
They also attend Toastmaster classes on weekends and volunteer at the Asian Women's Welfare Association by packing goodie bags and giving lessons to children.