Treat them like any other item (mostly). If there's an upgrade, then retire them and put them away for safekeeping.

You might want to consider hanging on to some lvl 187-200 items even if they're not an upgrade. If I remember correctly, the new LFDungeon system takes iLvl into account and I think the heirlooms are still counting as iLvl 1 items for that purpose. (They may have changed that, however).