Yeah all that stuff is standardized now. You CAN change the HTML, but it’s more complicated now needing custom functions and stuff. To change date formatting, you can do that right from your Admin settings though, it’s somewhere in there. Otherwise you can hide stuff via CSS and accomplish just about any design with all the hooks the output provides. If you really want to go old school and write you own comments loop, that is a possibility too. All this, and stuff on comment numbering, in my upcoming book =)
Here is the WordPress documentation on wp_list_comments.
It appears that to customize the comments you would want to do something similar to the article under "Comments Only With A Custom Comment Display"
You will then need to edit the functions.php file to create the custom callback.
I imagine, changing the "Date Format" under settings> general in the WP admin, changes the dates globally across your site (including comments)