Correct way of upgrading with huge data
Option 1:


Inventory your 3rd party modules as well as your custom modules. Identify those modules that has a Drupal 7 version or if it's included in Drupal 7 core.
Update the Drupal 6 core.
Update the contrib modules and themes.
Disable contrib modules.
When everything is up-to-date and contrib modules are disabled, run update.php. This process will take time depending on how big is your database.
Enable your modules and start fixing some API that is not supported by Drupal 7. Use coder module to assist you.
Check, check, check.


Option 2:


Build from scratch.
Use migrate module to migrate you Drupal 6 db to Drupal 7.