Depending on the brand of system you may not have to change the propane ECM but you will have to change all of the other components to run CNG. CNG is high pressure, LPG is low pressure so all the tanks and lines are incompatible.
Right now you can only get the UT tax credit with EPA certified kits, which means that unless you convert a crown vic with a BAF system you won't get an EPA kit for a passenger car in 2008. Baytech somehow says they have certified kits but the only platforms of their that I have seen with current EPA certs are the GM 6.0 and 8.1 liter vehicles 14000 GVW or higher; pretty much takes all of our passenger vehicles out of the loop.
There are some 058 and 06 certified kits available but not very many.
?Here?s to the hero's that Git-R-Done!!?