Here is how to do it. Enter some sample code into worksheet, e.g.
select * from dual
Highlight it, and hit "To upper/lower/init cap" button repeatedly to adjust the case as required. The completion insight is tuned to this setting.
Unfortunately, this setting is lost after restarting sql dev. Moreover, as Glen pointed out, it would be more logical to have insight case set from formatter preferences. This has been fixed for 18.2.