Can I switch to a different courier later with the Shipping Carriers plugin?

Yes, the choice can be changed anytime. Go to Settings → Store Settings → Shipping Setup and scroll to Shipping Carriers. Untick the old courier, tick the new one and click Save carriers; your plan sets how many you can have at once. Then click Configure on the new carrier, enter its API details, choose the Mode (Test / Sandbox or live) and switch it on. A carrier cannot be enabled until its keys are saved.