Aridsoul and Ranaishaq: (Following is only my experience, maybe some over here might be more lucky)
Ok so my experience was not really good. It is very hard to get it (the old 6 digit number) converted to new Khi number, specially if the car token has been paid in any province other than Sindh.
My first car was a Fiat 124, it was Khi reg (6 digit), the owner moved to Rawalpindi back in 1977, so then token too was paid here since then. I bought the car in 1993, and continued paying token here. After keeping for few years I learned that I need to get it changed to new number. So, when I contacted ETO Khi, I was informed, that I need to pay the token starting 1977 till 1998.