You can change your service plan at any time by going to the Profile page within your SingleFeed account and selecting the "Service Plan" link. A popup with explanations of the plans available to you will appear. When changing plans mid-month, fees will always be prorated.
SingleFeed currently provides three service plans:
Starter: Limited Engine submission - Self Service
Gold: All Engine submission - Self Service
Premium: All Engine submission - w/ Personal Account Manager