Many older WP sites used categories and/or pages to organize what were essentially subsites without proper access control and other site-specific goodies like themes, widgets, and plugins.
This plugin lets you move that content to another site on your WordPress network with a single click, with seamless redirects.
I needed it on www.robadadonne.it when moving old contents from root domain to subsites.
This is beta software. Back stuff up first.
This plugin is forked from https://wordpress.org/plugins/move-to-subsite/
Please contribute on Github https://github.com/palermomarco/move-to-subsite