Hi DeclanDonnelly,
You are going to install SEP 12.1, It's using .slf files to activate the product.
You might have receive email notification from Symantec to upgrade the proudct, email might contain new serial number to download SEP 12.1.
To avoid future problems make sure you have valid number seats against purchase licensed. If you overdeployed clients SEPM will start reporting over deployed status.
If you have correct number of seats then you are good to go.