How to Change Vector Datasource in QGIS 3.2.0?

Update: changeDataSource plugin has been updated for QGIS 3.x.

In qgis 2 this functionality was not native but came from a plugin (changeDataSource), it seem this plugin has not been ported to qgis 3. You could try to contact the plugin creator to made this happen (by sponsoring the upgrade or helping with the migration...)

You could also propose for this fonctionality to be added in qgis core (again more likely to happen and/or quicker if you sponsor it or contribute)