Can I change plan later?

Yes, in Settings, Billing. Upgrading takes effect immediately, so the extra stores, seats or Insights unlock the moment you confirm; downgrading takes effect at the end of the period you have already paid for, so nothing you paid for is taken away early.

Nothing about your data changes either way: costs, rules and history stay exactly as they are, and features simply lock or unlock. What each plan includes, side by side, is on the pricing page.