Back in 08, I had a little issue.
When going from Wordpress to Drupal 6, I had to go to Drupal 5 and then upgrade to Drupal 6.
I friend of mine did this recently, and did not encounter any major issues.
If you have tons of content, comments and hacks, you'd better find a good programmer with experience in MySQL and Drupal to help you with the migration. It was a pain for me when I used the Wordpress > Drupal migration module. Comments had to be reassigned manually and recalculated in terms of votin API.
Other than that, everything else went great. Since then, I've never looked back to WP. All our site are entirely developed in Drupal. There is simply no comparison.
Wish you luck
Great choice with Drupal its a fantastic system,well if you really have that much content you will need a good Mysql programmer to help you with the transition,i'm sure it will be a pain in the ass
Drupal has a better "out-of-the-box" Website architecture than wordpress. So with that regard, Drupal is a better choice to build a large article site due to some of the feature such as the navigation blocks and taxonomy menus. This is not to say that drupal is better. With some modifucation, wordpress can do the same thing.
The big advantage with wordpress are the plugins. Drupal isn't so easy and may require the assistance of a programmer.
A big issue with Drupal is running adsense. You can't place adsense directly into drupal due to some conflicts. Either you will need to place the adsense into blocks which is crummy for ad placement or to use some modules and create a node to insert the ads directly into the content.
Well i must agree with bogart on that one,but i also want you to take into account that Drupal isnt a blogging script in the first place,its a CMF,and it really isn't for rookies (myself included) because it's just a system that has a lot more to offer then Wordpress or Joomla,but with knowledge of course,you need to be a good programmer to use Drupals full potential,but even without advanced coding it can be used to a good degree if you combine some of the various modules that exist for it.For example Drupal is a great platform for a social-networking site etc...the possibilities are endless,but if you want a simple blogging platform,nothing beats Wordpress