I agree that uploading is copying-then-death. I think you're basically correct with your thought experiment, but your worries about vagueness are unfounded. The appropriate question is what counts as death? Consider the following two scenarios: 1. A copy of you is stored on a supercomputer and you're then obliterated in a furnace. 2. A procedure is being performed on your brain, you're awake the entire time, and you remain coherent throughout. In scenario 1 we have a paradigmatic example of death: obliteration in a furnace. In scenario 2 we have a paradigmatic example of surviving an operation without harm. I would say that, if the procedure in 2 involves replacing all or part of your brain, whether it is performed swiftly or slowly is unimportant. Moreover, even if you lost consciousness, it would not be death; people can lose consciousness without any harm coming to them.

Note that you can adjust the first scenario - say, by insisting that the copy is made at the instant of death or that the copying process is destructive as it transpires, or whatever - but the scenario still could go as described. That is, we are supposed to believe that the copy is a continuation of the person despite the possibility of inserting paradigmatic examples of death into the process. This is a clear case of simply stipulating that 'death' (and 'survival') should mean something entirely different. You can't hold that you're still speaking about survival, when you insist on surviving any number of paradigmatic cases of death (such as being obliterated in a furnace). There are no forms of death - as we ordinarily conceive of death - that cannot be inserted into the uploading scenario. So we have here as clear a case of something that cannot count as survival as is possible to have. Anybody who argues otherwise is not arguing for survival, but stipulating a new meaning for the words 'survival', 'death', etc. That's fine, but they're still dead, they're just not 'dead'.

I think this realisation makes understanding something like the brain transplant you describe a little easier. For we can say that we are living so long as we don't undergo anything that would count as dying (which is just to say that we don't die). There's nothing mysterious about this. We don't need to go looking for the one part of the body that maintains our identity under transformation, or start reifying information into a pseudo-soul, or whatever. We just need to ensure whatever we do doesn't count as death (as ordinarily conceived). Now, in the case of undergoing an operation, there are clear guidelines. We need to maintain viability. I cannot do certain things to you and keep you alive, unless I perform certain interventions. So I think the answer is quite simple: I can do anything to you - make any change - as long as I can keep you alive throughout the process. I can replace your whole brain, as long as you remain viable throughout the process, and you'll still be alive at the end of it. You will, of course, be 'brain dead' unless I maintain certain features of your nervous system too. But this isn't mysterious either; it's just that I need to maintain certain structural features of your nervous system to avoid permanent loss of faculties (such as motor control, memory, etc). Replacement with an artificial nervous system is likewise unproblematic, as long as it maintains these important faculties.

A lot of the confusion here comes from unnecessary reification. For example, that the nervous system must be kept structurally intact to maintain certain faculties, does not mean that it somehow 'contains' those faculties. You can replace it at will, so long as you can keep the patient alive. The person is not 'in' the structure (or the material), but the structure is a prerequisite for maintaining certain faculties. The common mistake here is thinking that we must be the structure (or pattern) if we're not the material, but neither claim makes sense. Alternatively, say you have a major part of your brain replaced, and the match is not exact. Somebody might, for example, point out that your personality has changed. Horrified, you might wonder, "Am I still me?" But this question is clearly absurd. There is no sense in which you could ask if you are still you. Nor can you coherently ask, "Did I die on the operating table?" Now, you might ask whether you merely came into existence on the operating table, after the original died, etc. But this, too, is nonsense. It assumes a reified concept of "self" or "identity." There is nothing you can "lose" that would count as a prior version of you 'dying' and your being born anew (whether slightly different or not). Of course, there are such things as irreversible mental degradation, dementia, etc. These are tragic and we rightfully speak of a loss of identity, but there'd be no such tragedy in a bout of dementia. A temporary loss of identity is not a loss of identity followed by gaining a new identity; it's a behavioural aberration. A temporary loss of identity with a change in temperament when one recovers is, likewise, unproblematic in this sense; we undergo changes in temperament regardless. Of course, extreme change can bring with it questions of loss of identity, but this is no more problematic for our scenario than an operation gone wrong. "He never fully recovered from his operation," we might say. Sad, yes, but this type of thing happens even outside of thought experiments.