You won't be able to use unused CAL's for Exchange Server 2010 with Exchange Server 2016.
"Purchasing CALs for earlier versions of an application does not grant the right to use later versions of the application. However, purchasing CALs for later versions of applications grants the right to use earlier versions of that application. For example, a CAL purchased for Windows 2000 does not grant the right to use Windows Server 2003. However, a CAL purchased for Windows Server 2003 grants the right to use Windows 2000. This is true with other applications as well, such as Exchange Server and SQLServer. "