Yes, you can upgrade at any time and you are invoiced a prorated amount.
For EventPilot subscriptions: EventPilot subscriptions are set to not automatically renew. Upgrading does not change this setting. You will only be charged the prorated amount for the current subscription period.
For ScanHunt subscriptions: The subscription renewal date does not change. ScanHunt subscriptions auto-renew monthly. The next renewal will run on your upgraded tier.