Hi,
Problem is that the activation key will be for new license only and it can overwrite the existing license. The same is written is Anyconnect Faq license page , I am pasting it below also:
Q. What happens to my older AnyConnect licenses when I install the new licenses?
A. All older AnyConnect licenses will be removed and replaced by the new licensing model when you install your new Product Activation Key (PAK).
Further, Anyconnect license with ASA are honor based, initially when you install the license. It enables ASA to support VPN users to maximum capacity.
Q. How do the new licenses work with the ASA?
A. You will receive a multi-use product activation key per Plus or Apex license purchased. This multi-use product activation key gets activated on each ASA at www.cisco.com/go/license. After activating the key, the ASA is unlocked for its maximum hardware capacity. Complying with the unique/authorized user counts and term limits are honor system and are not physically enforced by the ASA or AnyConnect. If you purchase more than one Apex license or a Plus and Apex license, we ask that you register each PAK to each ASA, although doing so does not change the resulting license key generated for the ASA. This is to ensure that if you open up a support case in the future that there is a record of your license purchase.
Source:https://www.cisco.com/c/en/us/support/docs/security/anyconnect-secure-mobility-client/200191-AnyConnect-Licensing-Frequently-Asked-Qu.html#anc30
