I have a 6310i with a 91 Car Kit. I believe Nokia are replacing this phone in a few months. Can you buy a new cradle only that fits the new phone or do you need a whole new car kit?
If you're thinking of the 6230, I believe as with all the nokia handsets with the new connector, the only wired OEM carkit its compatible with is CARK-126.
Unfortunately the black box in CARK-91 is different to that in CARK-126, so using a newer cradle with the old box would not work.