Copy texture from one channel to another

To achieve this I utilize Anchor Points. Start by applying your texture to the desired channel, such as base color. Then, add an anchor point to this layer and give it a name of your choice. Next, create another fill layer with the target channel active, for instance, the roughness channel. Instead of applying again the same texture, apply an "anchor point" and select the one you previously created. Make sure to set the referenced channel to the one where you originally applied the texture (in this example, it's the base color channel).