I've recently started using ObjectDock (immediately upgrading to Plus)
OD free needs to be uninstalled before you install ODPlus. If you didn't do that, save the theme you've created, make a copy of whatever icons, docklets and backgrounds you've added and any themes you've saved, uninstall the plus, uninstall the free if it still shows, reboot, re-install the Plus.
When you copy the icons back, you need to put them in the same place they were before or else the themes you've saved won't find them and you'll have to change the icons again. If you had the icons in a Non-Object Dock folder (i.e. you put them somewhere besides a directory created by Object Dock) it won't be a problem, OD won't delete them.