Dear Shali,
As regards LPG conversion center, please note down following contact numbers. You may call them directly and visit their office for further information and LPG conversion. Their office is located near Baloch Colony flyover:
Mr. Muhammad Saeed
General Manager
TESLA Industries
Cell: 0321-5055521
As far as I know, they do not exchange CNG kit with LPG kit. However, you may post an add on OLX.com.pk. There may be an offer from a buyer who still thinks CNG a suitable fuel despite knowing the fact that CNG has availability issue and there are ques of dozens of cars waiting for CNG filling.
At PAF Air Base, Shahrah e Faisal, Karachi there is HASCOL fuel station. From there you can get LPG filling (similar to CNG filling through valve into the LPG cylinder). The price as on today (10/12/2012) of 1 Liter of LPG is Rs.75.41.
Regards,