And I suspect magic begins when this mind substrate is extracted from the rest of the body while continuity of consciousness is intact and then integrated with a larger substrate with its own energy/material configuration. This is one plausible way to achieve immortality.

With this new substrate, the person will be able to do things with the mind one would ordinarily be unable to, such as teleoperate a remote unit or exist in a virtual world.

Would it mean the threshold of mind uploading is met if the machine portion is greater than the biological side?

What will it look like when the machine side gradually overtakes and completely replaces the biological side? I suspect the person will continue to maintain continuity of consciousness.