How long does it take for a service upgrade to be applied after upgrading subscription?

How long does it take for a service upgrade to be applied after upgrading a subscription? I am wanting to use the basic safety plan for a short period, while I learn how to use the device's messaging and live tracking features, before upgrading to the Expedition plan for my trip. I am concerned that the service upgrade might only be applied at the end of the billing period - which would be well after my trip has started. Advice?