You can turn any add-on car into a replacement car but it is more work. When adding a car (newer cars), you only have to drop the dlc file into the folder and add a line in dlc list. When replacing a car, you have to manually drop it in the newest dlc (patchday something) in order to overwrite the model. You most likely have to edit vehicles.meta, handling.meta or more. To convert a "add-on" car into a replacement, you have to go into the dlc file that the car comes in. Then you have to extract all the files necessary for replacing.
You want to replace the buffalo with a dodge charger. The dlc-file that the mod comes in might be called "charger". You must open the file in OpenIV and then enter the different locations. For example, going to update\update.rpf\common\data\levels\gta5 and then extracting the vehicles.meta, then copying the lines necessary and pasting it in the original vehicles.meta. This must be done for vehicles.meta, and might be necessary for handling or more advanced also (working dials, components, liveries).
This is the basic process:
Go into vehicle dlc file and find everything you need (vehicles.meta, handling, etc.) > extract them > add them to the original files in your mods folder > add car files (ex. buffalo.ytd, buffalo.yft, etc.)
Hope that helped, I'm happy to help if you need help, just send a reply.