I’ve not used the @bobray UpgradeMODX extra but his extras are usually excellent.
Other automated options are the brilliant SiteDash … and if your host has software management features such as Softaculous, you can import your installation and upgrade from there. I find both of these methods extremely reliable and both will backup database and files.
I don’t know if anyone has tried it, but I’m fairly confident that UpgradeMODX will work to upgrade from MODX 2.8.4 to MODX 3.0.0, and then on to MODX 3.0.x (it’s important not to skip 3.0.0).
UGM definitely works to upgrade from 2.8.3 - 3.0.0.
Just install it on the 2.8.4 site, change the ugm_show_modx3 System Setting to “Yes,” and launch UGM from the dashboard. Then install it on the 3.0.0 site and launch it again. Clear the cache and your browser cache if you see problems after an upgrade.
Important: the core directory must be under the web root for MODX 3 (nothing to do with UGM – just a requirement for MODX 3). If you need to move the core directory back under the web root. Test thoroughly to make sure the manager and front end are working before doing the upgrade.