They'll need to purchase a license with the new hardware. You'd have to contact the Meraki AM to see if they can swap licenses, but its unlikely (unless they're a recent purchase, in which case they may agree to it for a credit).
Unless they customer has moved to the Per Device Licensing model, they will be on the Co-termination licensing model. In the Co-termination model if they purchase a 1-year license for the new hardware then licenses will 'flow back' from the SM licenses to the point where its equal (think of the licenses as a bucket of tokens spread across all devices).
The renewal date will no longer be 5 years away, and depending on the product(s) purchased it may only be a year or two out from the date of the new hardware purchase, but it effectively pulls the licenses from the SM to the new hardware. Have a look at this tool to see if it can help you calculate the impact it may have, https://documentation.meraki.com/General_Administration/Licensing/Using_the_License_Calculator.