It is possible to transform raid-appropriate gear into tier pieces by using the Creation Catalyst (either in Zereth Mortis or Thaldrazus). When I play Paladin, and a Warrior chestpiece drops from Sepulcher of the First Ones, I can convert it into the Paladin tier piece without paying extra.
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Additionally, this system works on select Forbidden Reach gear (based on its item level), which is probably a good thing since soloing even the Sepulcher of the First Ones is impossible at the current power level of our characters.
In raids that contain current content, legacy loot rules do not apply, so fewer drops (and fewer chances of getting the appearance you want) make collecting transmogs a time-consuming process. In 10.1.5, you will be able to solo or duo Sepulcher of the First Ones to gain gear that can be converted to tier items.