Can I switch tiers mid-month?

Yes: moving up from Starter to Pro, adding a line, going multi-line — the change takes effect immediately and billing adjusts pro-rata for the current month, unlike cancellation. Moving up cuts nothing: settings, scripts and history stay; only capacity changes. Moving down a tier also works, taking effect at the start of the following month to avoid cutting a service in full use. Everything happens from the dashboard, Billing section, without negotiating with anyone.