The official post provided a pretty in-depth breakdown of just how this system will work. Basically, any and all armor – from Common to Gold – including gear sets will now be able to be altered via transmog. These Appearance Mods will let you craft your specific look. Like pretty much every transmog system, this will only affect cosmect looks, not stats or functional effects. Additionally, this system does not extend to weapons.
There are limitations, however. For example, you cannot transmog a non-exotic item to look like an exotic item. To take part in the system, inspect the gear. In the section where you would typically insert mods or a dye, look for “Mod Appearance.” From here, you’ll be able to select the desired look of your gear.
Note that appearance mods unlocked on one character will be accessible on your other Agents. Additionally, as expected, any changes you make to your appearance using this transmog system will be viewable not just to you, but to other players as well. As to when TU11 launches, we don’t have a specific date.