Currently Being ModeratedMay 3, 2012 11:42 AM (in response to Jellyby)
If you are selecting the new icon file and then pressing Command-C (Copy), that will not work to place the icon on the Clipboard (which is where it must be in order to do a Paste).
Do a Get Info on the new icon file. In its Get Info window, select the icon in the upper left, then press Command-C. Then go to the old file, do a Get Info, select itsicon in the upper left, and press Command-V (Paste).
If the Get Info window displays a generic icon for the new file in the upper left, look down to the preview area in the Get Info window - it should display the icon you want to snag. Double-click that preview image, and it should open in Preview. Then click the image in Preview once, press Command-A (Select All), then press Command-C (Copy). Close the Preview window, do a Get Info on the target file, select its icon in the upper left and press Command-V (Paste).