If customer purchased the 100mbps probe and later wish to upgrade to 1Gbps probe or higher. What is the process for them to upgrade?

Customer will still need to purchase a new probe rated to 1Gbps, and to also upgrade the subscription to 1Gbps. 

Note that if the customer is already planning to migrate to a larger link then 1Gbps probe may be the best option for them in the first place.