Having visited Drupalcon London and experienced the great community spirit once again, it was time to upgrade a few D6 sites!
Starting out with with the easier boran.ie and lepandevevey.ch, each site hat its own particularities.
New themes had to be made, and views recreated. Image handling required manual configuration.
As regards debugging modules (during migration or any time), a nice nugget found is https://ratatosk.backpackit.com/pub/1836982-debugging-drupal.
Debugging changed APIs (Converting 6.x modules to 7.x): http://drupal.org/node/224333
The general procedure was:
Copy the D6 DB, disable all modules and non default themes. Enable zen theme.
Run update.php, if this fails debug (and re-copy the D& DN and start again if needed). Sometimes D6 tables needed manual massaging.
Enable bartik as the main theme and seven for admin.
Check the watchdog log, enable each new D7 module one at a time and test, keep checking the log.
Themes were not migrated but new themes created, based on Bartik or Omega. Drush was used to create a repeatable list of commands for the above steps, to ensure the iterations of DB migration were consistent.
I'm currently in the middle or a much more complicated site migration, more on that another time..