There was a Rogue Trader in the Eisenhorn trilogy (iirc) that had done the same, replaced almost all of his body with machinery except his head. And his entire crew was servitors. So that he could feel closer to the ship.

There wasn't any malevolent intent, but it was still basically a heresy since he wasn't a tech priest.

Yeah in the Eisenhorn trilogy there is a villian who does a complete conversion, partly via sorcery. There is a magos who has all but his brain replaced. And there is a rogue trader who is just a head on a golden body. This is in addition to the various less invasive cybernetics that various characters pick up as time goes on.

I would say barring special circumstances, you can replace almost everything, but you still need a small chunk of brain. For tech, religious, and other reasons.

Well its an old necro but I may as well add that there's a heretical techpriest in the Priests of Mars trilogy that is a machine powered by combining brains of other techpriests in jars into one conciousness. He's not fully mechanical, but the other tech priests consider it AI level tech heresy even though he technically still uses brains.

Human consciousness(without a brain) into a machine might be tech-heresy(disembodied Tech Priest Grey Knights book). However if it's a piece by piece process it's not. If the consciousness is based on a human brain even if a digital copy still it's ok. Cyber resurrection, (DH Inquisitors'Handbook) the full resurrection options reads like only a digital copy of consciousness is used.