For now it's only for fresh installs.

"TinyMCE is a powerful rich-text editor that allows users to create formatted content within a user-friendly interface.

The popular editor is the default editor in new installations of Moodle 4.2 onwards and can be made default in upgraded sites from Administration > Site administration > Plugins > Text editors > Manage editors.

If enabled, users may select the TinyMCE editor from the User menu > Preferences > Editor preferences."
https://docs.moodle.org/402/en/TinyMCE_editor