In Intellij, how do I toggle between camel case and underscore spaced?

Two plugins offer this feature:

I use a plugin called String Manipulation which does what you need (and more).

Select historyOfPresentIllness and press Alt / option+M to bring up the plugin menu, then press:

• 5 – To snake_case (or to camelCase) which converts to history_of_present_illness
• 6 – To hyphen-case (or to snake_case) which converts to history-of-present-illness

To make this easier, you could set up a shortcut at File | Settings | Keymap.

There also is the CamelCase plugin.

SHIFT+Alt / option+U toggles the selection between formats:

historyOfPresentIllness –> history_of_present_illness –> HISTORY_OF_PRESENT_ILLNESS –> HistoryOfPresentIllness –> historyOfPresentIllness

You can also undo your changes (now that a bug in the plugin got fixed).