You can drag collections across libraries. You can select all items and copy them. You just can't (currently) copy all collections at once.
If a collection has been previously dragged, you can delete it in the target library and drag again to update it, or you can just drag all items in that collection in the source library to that collection in the target library and Zotero will automatically add any previously copied items to that collection.
Upgrade Storage
If a collection has been previously dragged, you can delete it in the target library and drag again to update it, or you can just drag all items in that collection in the source library to that collection in the target library and Zotero will automatically add any previously copied items to that collection.