I'd like to downgrade my reserved events for a data category (ex. errors, spans/performance units, etc.) but it's not clear to me when the changes will be applied.

Reserved capacity is purchased for the current billing cycle if you’re on a monthly plan, or for the annual cycle if you’re on an annual plan.
It cannot be refunded or downgraded mid-term. Downgrading will reduce your reserved capacity for the following billing period.

Downgrading your reserved quota or subscription can be done from the Subscription Management flow: https://sentry.io/orgredirect/settings/billing/checkout/