All the data in your account will be safely stored to your account for as long as you continue this account. Whether you upgrade or downgrade your account does not affect your data.
However, up-/downgrading does affect the access you will have to that data. So you might not have access to some of your data after you've downgraded your account. But don't panic, as soon as you upgrade your account, you will have full access to it again! : )