change group ownership specifying from to for entire file system

I have 3 specific user accounts (less than 10 anyway), for all files & folders under a specific /data or /home directory, I want to change just the group ownership of all occurring files/folders of those specific users. I don't know where everywhere might be (besides /home and /data) so I want to do a -R on /. Is there a way to do that? The existing group is named XYZ and I want to change all files & folders that are owned by ron.XYZ to ron.users. How can that be done?