How can I find all database schema changes introduced in a CiviCRM major version upgrade?
One way to do it would be to pore through the CRM/Upgrade/ directory in the new version, looking for schema changes. However these are distributed across multiple files, depending on which alpha or beta version the change was introduced in. E.g. the next_sched_contribution_date change was introduced in this file:

civicrm/CRM/Upgrade/Incremental/sql/4.4.alpha1.mysql.tpl