Do you create a new instance every time you awaken from sleep? If not, why not?
Because the same you has been active the whole time. Your brain didn't cease function through sleep. Kind of like sleep mode vs. rebooting a computer.
You're mostly driven by bodily conditions and hormones. A computer recording of you isn't going to behave the same because it has no particular motivation to behave any specific way in the first place.
If you can simulate a brain, you can simulate hormones.
How do you decide what to set them to? You don't have intrinsic motivations anymore!
Eh? You can have as many intrinsic motivations as you like. You just copy the procedure from the original brain.
The original ones are wrong though (well, not adaptive). Hunger is pointless when you can't eat.
Hans Moravec has an incrementalist approach to this which I think actually does accomplish eliminating the copy issue.
To be noted, if you accept a brain upload made gradually you should also accept a brain upload made discontinuously. If the same brain state comes out, the process can't actually matter.