The entered user credential must be a Global Administrator to add, remove or modify licenses. For more details about assigning roles,refer this Microsoft article
In the evaluation version, you can modify licenses for 50 users at a time. There are no other limitations in the evaluation version.
The two options available in the License changes step are:
Using License Reports section in the Office 365 Manager module, you can view all the information about the licenses owned by the Organization and the licenses assigned for each user.
If Add option is selected, the selected licenses will be added to the existing licenses for the selected user(s). If Replace option is selected, the existing licenses will be removed and new licenses will be assigned for the selected user(s).
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