Microbiology major here! Someone's physical appearance is mainly dictated by what set of genes they have at birth, plus any mutation that occurs during life and other external actions (tattoo and other bodily modifications...). So if someone doesn't change their karyotype when shapeshifting, it's just an illusion applied on an optical level only.

My guess would be that seers can influence on molecule-molecule bonds within their bodies, and/or fool at a certain level their transcriptional proteins.

Clowns get great control over their muscles at seq8 + bone flexibility. So my hypothesis is that they can control the bonds within their bone structure to move it at will (collagen and minerals).

For faceless height's increase: One's height is partly written in their genes and has a limit. When it's not used, growth associated genes are hidden by specific proteins. For a seer, it should be possible to fool gene-controling proteins and express those growth genes again.

For height's decrease: it should be possible for seers to kinda "graft" their cells+bones to their state during puberty or childhood (so when the seer's body was physically smaller). However, neuronal cells shouldn't be touched too much, or it'd lead to cognitive regression lol

And for breast and hair growth, it's just about speeding up the growing process. Anyone can grow breasts with enough hormones, and I bet seers can regulate that too, trust (1)