Hi,
With no more information, i would be tempted to answer you to just backup your courses from your different Moodle sites, and restore them all on the same Moodle you want to keep, having a version at least the same as the greater of all different Moodle version used...
But perhaps you question is more complex in reality, if you have to deal with several authentication modes, plugins used, not the same in all Moodles.
Séverin