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.