If I start small and later outgrow my plan, can I upgrade without losing anything?

Yes, and it's painless by design. From the Subscription page you can change plans whenever you like; storage upgrades apply instantly and no data is lost when you switch. Snaptide even makes a point of saying there are no locked tools on lower plans, so upgrading is purely about gaining capacity. The one thing to know is that if you switch before your billing period ends, the unused time on your old plan is converted into credit toward the new one, capped at the new plan's price — which is a fair enough way to handle it.