Just a few notes. Don't uninstall GarageBand '11, but keep it in addition to GarageBand 10. Some features have been discontinued in GarageBand 10.0. You may need the older version for your older projects, if you use the sound effects instruments or want to make a podcast.
Both versions are working with MacOS X 10.9.
You'll notice,that GarageBand 10.0 will only install a basic set of instruments and loops. More instruments can be bought by an in-app purchase.
See: Expanding GarageBand for Mac (v10.0) content with an in-app purchase
You may be able to get this purchase for free, since you owned a previous version: Try "GarageBand > Restore Purchase".