Some Rules
All Upgrades take place immediately. You get to use your new plan, its features and any credit token uplift, as soon as you confirm your changes
All Downgrades take place at the end of the billing period. This means you get to use all your current account features right up till the end of your account period.
If you downgrade and you had a survey that used features that are now only available in a higher version, you cannot access that survey and it will be Closed and Archived. If you need to get access to that survey, you'll need to upgrade to a version that has those features.
If you downgrade to the FREE account and you had spare/excess Response Tokens they will be removed and you'll have a maximum of the basic available tokens allowed on a Free account.