That mauy be a tough question about contract here, seems the support team know very little about contracts and/or plans.
The Family Plan is basically the first or primary mobile device, this then allows you to administer any additional phones you may add at a later date. You can get discounts when you are the family plan administrator but good luck with that.
As for changing plan, you can do this in your Virgin Mobile account and it should give you details of whether you are in a rolling thirty day contract (end of agreed term you automatically go in to this) or a Flexible (Freestyle?) account where you can change on a monthly basis to suit your needs.
Good luck, hope i helped a little.