⚔️ You can transmog armor with Yoana at Crow's Perch after the Master Armorers quest.⚔️ You can transmog swords with Hattori in Novigrad after the Of Swords and Dumplings quest.⚔️ Any armor or sword Geralt picks up unlocks as a skin.⚔️ It costs 100-200 coin every time you swap skins.
One of the big changes in The Witcher 3 Remastered is that you can now transmog armor and swords, swapping their skins at will.
You can't transmog at every blacksmith, only the two master smiths in the game, Yoana at Crow's Perch, and Éibhear Hattori in Novigrad, who will let you reskin armor and swords, respectively.
First, though, you have to complete their quests:
Master Armorers
Speak to Fergus Graem, the dwarf armorer at Crow's Perch, and ask about mastercrafted armor. Then Yoana will ask you to travel to Undvik in Skellige to recover some blacksmith tools.
Of Swords and Dumplings
Speak to Éibhear Hattori at his shop in Novigrad, northwest of Glory Gate, to start his quest and unlock him as a blacksmith.
Once you've unlocked Yoana and Hattori, simply choose the "reforge" tab on the top right when purchasing or crafting with them.
A couple more quick things about transmog:
Transmog armor dyes the same colour as the base armor.
You unlock new sword and armor skins when you place them in Geralt's inventory.
🦅 Griffin: A medium set for signs🐈⬛ Feline: A light set for glass cannons🐻 Ursine: A heavy set for high defence🐺 Wolven: A medium all-rounder set
Unfortunately this doesn't seem to be retrospective, so for endgame saves, unless you've kept every armour piece, you'll have to acquire them again, or at least take them out of your stash and place them in your inventory to unlock them—not the best system I'll admit, hopefully this will be patched.
Despite being a smith who forges grandmaster witcher gear, for some reason Lazare Lafargue in the Blood and Wine DLC can't transmog armor either. Again, fingers crossed this gets patched in.