Blizzard has announced that Heirlooms will be Warbound when War Within launches, allowing you to send enchanted and gemmed Heirlooms to other members of your Warband.
Upon further consideration, we’d like to continue to support the playstyle of setting up heirlooms with enchants and gems, then passing them to alts as needed. Given the 11.0.0 patch is today, we can’t easily fix this in the current patch, but it will be changed back in the upcoming 11.0.2 patch.
While unfortunately your gear will become bound if you enchant/equip until the next patch, once it comes out, your heirloom gear will revert to Warbound, allowing you to again pass it between alts after equipping / enchanting it.