You gotta know your locations well. If a line was constructed to pass through Somerset, Orchard, Novena and Toa Payoh, why do we need a new line for? Orchard Road area does not have space for another underground line. Contrary to popular believe, Orchard MRT is diagonal to Orchard Road, pointing towards Isetan Scotts. Think about it. How do you extend an already congested underground area?
As for the converted stations, I believe you are refering to North Point and Lai King. Note that they are NOT located in the city centre. Space has actually been reserved at the side of the station as before they were interchange stations, North Point operated in stack platforms (like Bangkok's MRT and some of our future Downtown Line stations) and Lai King had space reserved above the station for a future realignment.
Not as easy as it looks now huh.