This is a typical software engineering problem. You have a legacy system, and what makes it one is actually data. You don’t want to throw away the result of almost 12 years of daily blogging.
This is the first constraint: I want to keep my data, but I want to migrate it to a new system.
URLs are the second constraint. You don’t change URLs without dire need. URLs are indexed by Google, URLs are bookmarked by users. Don’t ever break URLs or else bad things will happen 🙂
The same thing is true for images. Images are not part of an HTML page, they are referenced from the page by their own URL. Therefore the organization of images must be preserved or migrated as well, but that would involve updating URLs in more than 4200 posts. I would need to write a program to do the tedious work.
Other than that I’m free to do as I like 😄