replace all yellow with white

I have a fairly simple gif image that is black on a yellow background. I want to change it to be black on a white background.

In photoshop 6.0.1 (win) what is the easiest was to just replace all yellow with all white...?

Also there may be different shades of yellow where it blends with the black so it needs to deal with that as well...

Try using Image---»Adjustments---»Selective Color.

Without seeing the image, that's all I can really offer.

If the Black is BLACK, simply Desaturate to remove the colour information from the highlights.

As Phos says, we'd really need to see the image to advise further.

Change color mode to CMYK, split channels, and use the black channel only, using curves the bring the darkest spot to 100%.

Save for web and trash the yellows?