I would guess that there are hundreds of database changes between Moodle 2.5 and 3.4 (and that's just counting table definitions, not including and data transitions that take place).
If you want to see the differences, then there is a perfectly good tool designed for doing that - it is the Moodle upgrade process. Copy your database, upgrade it, then compare with the original.
If you have already configured your 3.4 site, then the only real way to merge the sites together would be to backup each of the courses from the old site and restore them to the new site. If the old site has a lot of courses and your new site has only configuration, then it will be quicker to just upgrade, then redo the configuration afterwards.