And why clearing cache is not enough?
Dropdown fields (and other simple option-based fieldtypes such as Radio Buttons, Checkboxes and Multi Select) are pretty dumb.
By design, whenever an Entry with a Dropdown field is saved, Craft simply grabs the selected value for that Dropdown field and stores it in the database for that Entry. Unlike relational fields such as Assets or Categories, there's no "connection" (i.e. relation) between the selected value being stored, and the Dropdown fields' option where the value was pulled from – this means that if you later decide to rename a Dropdown fields' option value from e.g.
"bar", the selected value for any existing entries will not change/update. In other words, any entries that had
"foo" selected, will still have
"foo" stored as their value for that particular field, even if that option doesn't exist anymore.
The easiest way to bulk update entries' selected Dropdown values is probably to execute an SQL query.
Something like the below should work (the following example would update the selected value for a field with the handle
"bar", for all entries that have
UPDATE craft_content SET field_myDropdownField = 'bar' WHERE field_myDropdownField = 'foo';
Oh, and don't forget to back up your database before running any manual SQL query, and to clear out your caches after you're done.