Yes, any time from Profile → Billing. Upgrades apply immediately and downgrades take effect at the end of the period.
Any time, from Profile → Billing.
Upgrading takes effect immediately, with a prorated charge for the rest of the current period. Your new document limit applies right away.
Downgrading takes effect at the end of the current period, so you keep what you paid for. If you're over the new plan's document limit when it kicks in, existing receipts stay viewable and exportable — new uploads pause until you're under the limit.
Most people start on Basic and move up when they realize how many receipts a family actually generates in a year. Dental, vision, prescriptions, and copays add up faster than expected.