Final Draft 10 / 9:
Note that the Replace Character function will correctly render the new character name in all elements.
If you don't like the results, go back to the Replace Character window and replace the new unwanted name with the previous name.
Final Draft 8 / 7:
First, since this action can't be undone via Edit > Undo, it's best to work on a test copy of your script. Go to File > Save As and give the script a different name.
If this gives you what you want, use this copy as the new master or go back to the master and do the Find/Replace of the character name in that one too.
Note that the Find and Replace function will correctly render the new character name in all paragraphs: