Actually, you'd have to wait at least 45 days after the cancellation of your current service to sign up as a new customer.
The free installation deals are to attract new customers. It costs money to not only pay the subcontractor to come to your home and perform the installation, but also for the parts that aren't included in the equipment lease, as you're only leasing the modem and radio, not the dish, the pole and all of the hardware required to install it. If the only thing required for the upgrade was swapping the modem, the upgrade would likely be free to existing customers as well.
HughesNet is no different in this than countless other companies that offer services that require professional equipment installation. New customers always get the best deals.