If your new logo includes a symbol that can be used alone, be sure to get separate files (.eps, .jpg & .png) of the symbol.
Always ask for various formats of your logo when hiring a designer. A vector file is most important (.eps), but ask for .jpg and .png files as well.
Ask for your logo in various versions when hiring a designer. A good, reputable graphic designer will supply your logo as full color, black & white and reversed in all formats (.eps, .jpg, .png as a minimum).