The upsell/cross sell block can act both as an upsell or cross sell depending on the setting used. The default option (cross sell) will add the product to cart. The upsell option will replace a specified cart product with the 1 offered.
Note how “The Compare at Price Snowboard” is replaced by the Videographer snowboard when using an upsell:
When using the upsell option the “Cart’s products” display rule must be added, to specify the product in the cart to replace.