Chosen Solution
Can I upgrade the HD in the surface pro 6
Unfortunately, the NVMe SSD in the Surface Pro 6 is soldered onto the motherboard. Hope this answers your question.
For the 128GB version, it is SK hynix HFB1M8M0331A (BC501) 128 GB NVMe SSD (see the teardown), and yes, it is soldered to the motherboard.