If you’re linked to an Affinity account, then you’re spot on - the subscription renewal is optional. If you are disconnected from the Affinity account at a later date and your account is graded as L or XL, then you would require a subscription at that point.
It’s worth noting that the Power User Subscription does come with some additional features which you would lose if it’s not renewed.
We’re rolling out the newer VAT return for cash accounting, so you should be migrated soon. If you’re on accrual accounting for VAT, you should already be on the new one.