There is no big change in heirlooms. Since 4.2. new heirlooms have been added to the list, you can now “upgrade” heirloom weapons by dealing in the old version and buying another version that can upgrade for 5 more levels. At the moment, heirlooms have two different level caps. The primary heirlooms will up only to level 80 while the heirlooms bought from guild vendors will up you to level 85. As the game appears right now, no heirloom will upgrade you all the way to level 90.

The new “upgraded” versions of the wow heirloom weapons will achieve level 85 as well, though you’ll need to buy the initial version and then
turn that into the vendor as part of the price of purchasing the upgrade. But be noticed, the “upgrades” are actually replacing rather than upgrading, you have to re-enchant the new ones.