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).
When you migrate to Office 365, there in one critical element in managing your Office 365 environment and that is your Office 365 License usage, because this is what burns your dollars based on the Office 365 license plan you have subscribed to. You need to get insights on how your Office 365 licenses are being put to use. Click here to know more.
I found Dockit product through SharePoint.com and found it extremely useful. We used Dockit to import hundreds of documents into SPS. Users were in desperate need for them but refused to take the time to upload them. Dockit saved us the upload time, and more importantly, made the information available in a timely fashion. Small issues were worked out quickly by a capable and dedicated customer support staff.
We find that Dockit is an extremely valuable tool for migrating intranet content to SharePoint Portal Server. When you are faced a large number of documents to migrate each of which has to be populated with metadata, Dockit reduces considerably the effort involved in publishing information. Typically it reduces the time a user has to spend per document by 75%, by enabling documents to be published in batch mode rather than real time through the standard SPS interface.
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We used Dockit to import over a thousand engineering documents, preserving the folder structure. It was simple to use and saved us a lot of time.