Uploading a .ucs file

From SOL10245:

Beginning in BIG-IP 10.1.0, when you restore the UCS file on the BIG-IP system, the UCS installer uses the following logic to determine whether to install the license from the UCS file or to retain the system's existing license file:

The system installs the license file from the UCS file only when the hostname in the UCS file matches the system hostname, even if the system already has a license.

So, if the hostname matches, yes it will be replaced. I always recommend keeping a backup of the license files for your BIG-IPs stored somewhere safe just in case. You can back them up by copying the file /config/bigip.license from the system before starting. It will contain the registration key near the bottom which you can use to reactivate if you have issues.