Depends on the company, but normally it works like this.
Get the insurance on current motorcycle, liab, or full coverage.
When you get something else later, you just call up the company, and tell them you want to switch a vehicle, give them the vin #, and you'll get a statement within 30 days telling you about the switch.
Shouldn't cost you anything to switch, unless you get a newer, more expensive bike, then of course full coverage will cost more. Just liability should stay about the same, because liab premiums are based on driver primarily not the vehicle.