I'm assuming you purchased something like this where XXXX is the firewall model number:
XXXX-NGTP
XXXX-NGTP-HA
In this case the cheaper license with "HA" on the end is identical to the first one, except that the firewall with the HA license must be a member of cluster with another "fully" licensed member. The HA-licensed firewall cannot be used just by itself or as a "cluster of one". The HA-licensed firewall can most definitely be converted to a fully licensed version; contact your reseller who will trade in the existing HA license for a credit, then charge you the difference for the full license version. The price discount on an HA license vs. the full license is usually about 20%.
Use of the term "standalone" in this context is a bit confusing, since that term generally indicates that the SMS function and gateway function are running on the same piece of hardware. An HA-licensed box cannot run in standalone mode and manage itself, but a fully licensed model can.
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