Hmm converting isn't too straightforward, but here's an overview.
Window classes: The biggest change is the introduction of anchors, so controls can resize automatically when the sheet is resized. If you don't need this then you can stick with the old 'bounds' approach. The introduction of Lua makes computed amounts (including some complex ones) more versatile, but I don't think you can use the previous mappings etc, so it kinda forces you to learn the scripting language.
Reference stuff: These are now in modules and managed quite differently. A module (.mod) is just a zip file (.zip) with the extension renamed, but it has a particular format (thumbnail icon, definition and data file). Try copying an existing module and renaming it back to .zip and have a look.
Campaign data: the DB file uses a different approach, previously you'd have <stringvalue name='mystring/> whereas now you have <mystring type='string'/>, so similar (and convertible) but different. Also, it supports nested values allowing structured types:
<ability>
<name type='string'>strength</name>
<bonus type='number'>3</bonus>
</ability>
You can then reference 'ability.name' and 'ability.bonus' from within the script. This looks odd, but it allows the code to be reused (and turned into a template) across multiple abilities (such as intelligence, education etc). The apporach is used extensively for skills.
Hope that gives you a flaour. You might also want to look at the xml documentation and modification guide found at the top of this forum.
Cheers
Stuart
(Foen)