Public User Signup option can not be changed when license limit has been reached

Under
User > User Signup Option
You have the option to "Allow people to sign up to create their account"
Once the maximum amount of users according to the license has been reached on the instance, it is not possible to make any changes to this section any more.
Instead, you get the following message presented:
"This Confluence installation is not licensed for additional users"

Workaround
Temporarily remove the "Can Use" permissions through the Global permissions in order to lower the used licenses of the instance under the max, change the options, re-add the "Can use" permissions.