From the MS Excel help I found this useful information:
Important When you break a link to a source, all formulas that that use the source are converted to their current value. For example, the link =SUM([Budget.xls]Annual!C10:C25) would be converted to =45. Because this action cannot be undone, you may want to save a version of the file before you start.
On the Edit menu, click Links.
In the Source list, click the link you want to break.
To select multiple linked objects, hold down CTRL and click each linked object.
To select all links, press CTRL+A.
Click Break Link.
If the link used a defined name, the name is not automatically removed. You may want to delete the name as well.
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