The operation was a success! Thanks to those who provided guidance throughout this thread and to Davidfor for supplying the link to the fimware image.
I replaced the original MicroSD card with one with an 8GB capacity and a speed of 30 MB per second. According to Wikpedia this is equivalent to UHS Speed Class 3 and refers to speed of writing rather than reading. However, I think my N647B might be operating a little bit more quickly and smoothly as a result. And obviously I have more internal storage after expanding the KoboEreader partition into the largest unallocated space.
It's probably also worth noting that I used a Phillips size 0 screwdriver.
The only problem is that the lower right part of the bezel is a little loose and makes clicking noises if I press it, but I have a feeling that it might have been like that before I dismantled the ereader.
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