It is quite easy to assign keyboard shortcuts in Oracle SQL Developer to actions in SQL Developer. I recently configured shortcuts to convert text to all UPPER and all lower case.
It is quite easy to change the keyboard shortcuts in Oracle SQL Developer. Just go to Tools → Preferences and navigate to Shortcut Keys in the left side menu. On the right there will be a lengthy list of actions to choose from. For converting to upper/lower case look in the Code Editor category. You will find Convert Selection to Lower and Convert Selection to Uppercase. You can then change the short to the desired key combination.