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.

For annual contracts, downgrades or reductions in reserved volume are applied at the end of the current annual term.

For monthly contracts, downgrades take effect at the end of the current monthly billing cycle. 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/, specifically Settings → Usage & Billing → Subscription → Manage Plan. If you need to adjust quotas or reserved volumes before the end of the current billing cycle or annual term, you can cancel your current subscription, which will move your organization to the Free Developer plan. After cancellation, you can resubscribe with the desired quotas.