ship of theseusing it
The mind is a dynamic, ongoing process, and so any attempt at capturing it as a 'snapshot' will inherently fail. It's like if you had a big box of ping pong balls being wiggled about so that the balls chaotically bounced against each other. You couldn't simulate the box of ping pong balls just by knowing the positions of the balls; you have to also know their velocity. You need the whole vector.
In practice, this means that the only way to achieve a successful brain upload is to ship of theseus it. The old organic brain and the new artificial brain must be tightly integrated so that the new brain can gradually take over functionality, at first duplicating parts of the original brain, which are then selectively shut off. At the end of this lengthy process, consciousness has transferred to the new brain. It's like any other transition; there has to be an in-between. You can't go from one place to another without crossing the intervening space.
Or at least that's what we think the skulljacks do to the people they grab, anyway. We've never captured an intact specimen so for all we really know they might just be slurping up their brain juices because the machines like the refreshing taste. So put your damn helmet on, rookie! I don't care about no back line, these things can come from anywhere.