The annual membership is only available for new members, that receive a discount compared to current monthly plans. It's available for the S plan for 29€ and the M plan for 59€.
This membership has a duration of 12 months ( 1 year), but the payment will be monthly. After the 12 months the membership will transfer to a regular monthly membership. If the payment fail the membership will be suspended.
Upgrades can be done only to L and XL plans (monthly prices), downgrades to the previous membership and pausing the membership isn't possible.