When I bought my 1/2 ton truck, I was able to transfer the plate from my car on to the truck. There was about 6 months left before it expired. All you do is pay the difference of what the truck plate would cost for 6 months.
Cheaper than just wasting the 6 months you already paid for on the car plate.
When it expires, they will send you a new truck plate and you will have to renew the registration. It will be the $58.50 that someone already stated.