If you combine two files that had links to some other files, or if you make a copy of your file which has links to other files, you will find that some formulae in this new file are linked to the old workbooks which you may have to delete and re-do the formulae. Instead you can directly point these links to the new workbooks through a few steps without having to re-do all the formulae.
Click Edit Links -> Change source, browse to find the current file which is open on your system. Click on the filename and click OK. All the links will be updated to the new file.
In Excel 2007, this option is available under Data -> Edit Links.