Upgrades can take place at any time by clicking the Switch Plan option on the Keeper's Society membership page.
If you decide to upgrade plans within the first five (5) days of your initial registration, you are within a grace period. The grace period allows Facebook to process your upgraded tier and the game to reward you the new items. Facebook applies the price adjustment between the two (2) tiers based on the prorated for the number of days left in your membership cycle.
If you decide to upgrade plans after the grace period, you will receive an in-game message to inform you of your new items. Upon clicking the "Upgrade" button on the in-game message, the game will auto refresh to ensure you receive your new rewards.