We don't know yet, but the technology was described as a tab where you click on the heirloom that you have unlocked, and one spawns in your bag. Since Blizzard usually try to reuse existing core technology solutions, my guess is that this "tab" is effectively a vendor that sells you heirlooms for free.
In that world, expect that enchanting works like it does today (eg: scales, with breakpoints), and that when you destroy an heirloom item with an enchant, the item and enchant are gone. Creating one from the tab on another toon would be like buying another one from the vendor.
I half suspect that they will retain, eg, the ability to mail heirlooms around so you don't *have* to destroy them and create new ones -- you just get the option. eg: this is a free "repurchase from vendor" that doesn't take up bag space, but you are not *obliged* to take advantage of it to get heirlooms on other alts.
(Also, obvs, these will behave like enchants on other items do, so if they remove them from a slot, the enchant will be gone.)
Note: all speculation, based on what Blizzard have publicly said, and my recognition of how their developers think based on working with similar developers for decades.