Following is what Microsoft have to said about Changing and Deleting a Define
Name.

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Change or delete a defined name  
On the Insert menu, point to Name, and then click Define.
In the Names in workbook list, click the name you want to change.
Do one of the following:
Change the name
Type the new name for the reference, and then click Add.
Click the original name, and then click Delete.
Change the cell, formula, or constant represented by a name
Change it in the Refers to box.

Delete the name
Click Delete.

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According to me, after renaming a Name, you have to manually change each and
every formula. After changing a name, I always use Find and Replace command
for changes to take effect.