ExpressRoute circuits are configured to allow you to burst up to two times the bandwidth limit you procured for no additional cost.
Scenario: It is estimated that during the migration, the bandwidth required between Azure and the New York office will be 1 Gbps. After the migration, a traffic burst of up to 3 Gbps will occur.
Reference:
https://docs.microsoft.com/en-us/azure/expressroute/expressroute-faqs