You can migrate MT over to WordPress easily – there’s an option for it in the WP admin area. You just need to export your MT files (from your MT admin) and then import it, based on what it is, to your WP area.
I know this works, because I used to have an MT blog, myself.
I can’t say *for sure* that it’ll preserve your current URL’s, but in all honesty, I think it does. Why? Because when I did the same thing with my MT blog, all I had to do was import it. Nothing else. And I lost nothing. All of my URLs were the same – as far as I can tell (and it’s been over a year since I did it) – no broken links to images or anything.
I *believe* it converted the URLs to my WordPress settings. so, to change your URL with your pretty permalinks, I think you can do what you want to do. Your example URL would be this in your permalink area in admin:
/archives/%post_id%
Now, I *don’t believe* that WP sets up post ID’s with leading zeros, or the PHP extension – so your post ID would probably be changed from “/archives/000100.php” to “/archives/100”. But you *may* be able to edit that by hacking a few things.
For the record, I’m only telling you this from my own experience. So take it with a grain of salt. Someone who knows better can come along and verify all of this, or simply correct me.
But I *do* know that, from my own experience, that conversion is not an issue – it was a snap. The URL thing is what you’ll have issues with, I think.
By the way, if I *am* right, you can simply use your .htaccess file to do a permanent redirect for your posts. How to write it exactly, I don’t know for sure – but I believe it can be done – that way you won’t lose any ranking (from my understanding – a 301 is what you’re supposed to do to prevent this) if that’s what you’re worried about.
Hope that helps a little bit!
