What should I do to change my predecessor's car into a rear drive or a four-wheel drive?

It's basically impossible for ordinary cars, so don't think about it. This time and energy are enough to buy an original rear-drive or four-wheel drive

But if you really want to change it, and your front-drive car series has a four-wheel drive version, it's entirely possible to convert it into a four-wheel drive. For example, the domestic Audi A4 2.T is the forerunner, but because A4 series has four-wheel drive models, for example, the imported A4 2.T Quattro is a four-wheel drive, so it is entirely possible (though not easy) to change the domestic A4 2.T into a four-wheel drive because of the common parts, but it will take a lot of effort to convert it into a rear drive.