I was in exactly the same situation.
I probably wouldn't bother re writing the site for data-assist
That link shows that you can change some things in the includes that will make the ADDT site work with 5.3 (but will break the dreamweaver addt links i.e. you wont be able to edit with menus etc)
As for learning WA, you will have to look at the support files in your login area, and follow the getting started guides. It is not ADDT, so you will have to learn something new. But I have been working with it for a few months now (but I do have a working knowledge of php) and like all things, the more you use it and have to solve problems, the more you understand it.