Photoshop offers both destructive and non-destructive black and white conversion techniques.
Destructive methods are ones that can't be re-adjusted afterwards. Once you've made the conversion, the only way to change it is to undo the conversion and start again.
Non-destructive methods use Photoshop's adjustment layers. The changes that you make to your photo are stored in a layer, and the original photo remains unchanged underneath. Then, when you're done, you flatten the image and it's as this stage that Photoshop makes all the changes to the photo permanent. You can edit the conversion at any time before flattening the image by clicking on the Adjustment Layer icon.