There’s no need to buy a payware cloud replacement addon. HDE v2 by Pablo Diaz brings you photo-realistic texture pack for free that will change the aspect of your Clouds (cumulus, stratus, cirrus), sky color, aircraft reflections and ground detail. Beware that this package is for FS9, but the cloud textures will work in FSX. Download here:
There is however a conversion of the sky textures from FS9 to FSX standard by Danny Glover available here:
Both packages requires manual installation, so backup the files to be replaced first (just make a zip of the textures folder)!