Originally Posted by
decibel08
...only wants to convert one or two of the cards ...
Provided you still hold a Green, Gold, Platinum or Centurion card the points balance and classification would be unaffected by closing or changing one card. However, if you have (for example) both a Gold and a Platinum card there isn't much benefit to converting the Gold card to another type. Typically better to simply close it and apply for another to earn the new account bonus.