Steps to reproduce:

1. Get yourself a playerhead in 1.12.2, e.g. with:

/give @p minecraft:skull 1 3 {SkullOwner:{Name:"Meri"}}

2. Put it e.g. in your hotbar.

3. Go into the current snapshot (17w48a), notice that the head is not called "Meri's Head" anymore, but "block.minecraft.player_headnamed".

This does not affect custom heads, e.g. try this command in 17w48a:

/give @p player_head{display:{Name:"Pink Frosted Donut"},SkullOwner:{Id:"b48503a4-6dec-438c-a3bc-6b5da7fb1fde",Properties:{textures:[{Value:"eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvODM3YzliODJiMTg2NjU2ZTlmNjM2M2EyYTFjNmE0YjViOTNjZmE5ZWY0ZGFkNmYxNmI5NGViYjVlMzYyNjc4In19fQ=="}]}}}

and it will give you a pink frosted donut head,
and it will also tell you

"Gave 1 [Pink Frosted Donut] to PLAYERNAME"

Custom player heads are successfully updated to "player_head" instead of the faulty "block.minecraft.player_headnamed".
For that I went into 1.12.2 first, packed my hotbar full with those custom heads, then I opened the world in 17w48a.

Also @Mods please do tell if I should open an own bugpost for this:
I used this give-command in 1.12.2:

/give @p minecraft:skull 1 3 {display:{Lore:["A strange Head Lore"]},SkullOwner:{Name:"Meri"}}

But the head only shows the default Alex-texture. If I place the skull somewhere though, it shows the skull owner's texture.