You can easily upgrade the board. All of your settings, styles, members, forums, posts, etc, would be migrated over to the 6-series.
You might want to grab the Trial Version (link on the left menu) and make a new install of it on your server so you can get the hang of the 6-series. It's quite a change from 5.x <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />