Our view is that the best way to upgrade existing Drupal modules is to contribute the work back to their Drupal.org projects.
For us, every project starts with goals. From goals comes strategy, from strategy comes planning (information architecture, interaction design, technical architecture) and from planning comes (a rather agile) implementation.
And for implementation, 2011 so far is the year of Drupal 7 — at least it has been for us, because all of the projects we've started this year have implementation powered by Drupal 7 at the core. In some ways, we love Drupal 7 so much more than Drupal 6, we don't like to look back. It's almost painful to have to deal with Drupal 6 anymore … especially when it comes to theming.