Q: Why does iCloud need 10gb more space now after upgrading to macOS Sierra

Photos doesn't sync to iCloud Photo Library anymore. It was prior to upgrade and now says I need 10GB more iCloud storage to enable. Why would it need more storage?

Your Photos library is probably already in iCloud.  If you want to enable a Photos Library again, tat is already in iCloud, Photos does not know that it is the same library with exactly the same photos already stored in iCloud. So it wants you to have enough storage to upload all photos from your library in addition to all photos already stored in iCloud.  Photos will not actually need the additional storage, because it will merge the library into the exististing library and avoid to upload duplicates, but the estimate for the storage required is a worst-case estimate.

the easiest way out would be to temporarily sign up for more cloud storage and sign out of the additional storage as soon as your library finished uploading.

Alternately you could create a new, empty Photos library and sync this library with iCloud. All photos currently in iCloud will download to your Mac.  If iCloud is storing your complete library it will be more economic to download from iCloud instead of uploading and merging.  The drawback would be, that the library you restore from iCloud will not contain your projects (books, calendars, cards, slideshows) and all named faces will be missing.