Hi Simon,
When performing a minor upgrade it should not be necessary to reactivate your license. The reason this is necessary during a major upgrade is because SmarterMail will not automatically see that your license is for the newer major version. However, because a minor upgrade keeps the same major version number and SmarterMail recognizes the same major version (e.g., 12.1 to 12.3), you shouldn't need to do any reactivating.
Hope this helps!