Lots of radio questions!
When you buy extra receivers, in order to control multiple vehicles with the same transmitter (not at the same time), you can change the receiver to match the transmitter.
Let's say you forget your transmitter at home (or it's broken) and you borrow or buy another transmitter which is on a different channel than your previous one. Obviously, the easiest is to change the transmitter, but that's illegal, so you change the crystals in the receivers you want to use with the new transmitter.
The main problem is that in North America, the air waves are so crowded, that RC is a SECONDARY user on our frequencies. The FCC is trying to protect the PRIMARY users who pay thousands (or maybe millions) for the right to use these frequencies. Since changing the transmitter crystal without re-tuning could cause the unit to interfere with industrial equipment on the PRIMARY users equipment, the FCC disallows it. Receivers don't transmit, so the FCC doesn't care if they are out of tune.
Does that help clarify things a bit?
Since you said you don't even have a car yet, before you order your radio, find out what channels (frequencies) are being used by others you might drive together with, and order your radio on a different channel. Either that, or get a radio with removable modules. Then you can buy additional modules for alternate channels.