If you change you plan to annual, then you are locked in for 12 months of prepaid service. You can then add on additional plans as needed, even for just a month if needed. Multiple plans can exist side by side until you disable each renewal.
In order to switch between accounts you need to Sign Out of one and then Sign In to the next. The only times Sign Out should be grayed out in your GoToMeeting menu is when you are in a session, or if you use a Mac and are already Signed Out MyMileStoneCard Login