If you have a lot of customization, this will not be an easy operation. NAV2013R3 has almost 1400 new or modified objects. Few very important codeunits are completely done from the beginning.
If you have not a lot of customization, I will highly recommend to upgrade on newer version before GoLive. This version has many better solutions.
You can use the same license (now, you can download only R2 license).
I upgraded all my NAV2013 projects to NAV2013R2. I wanted to have only one version to support.