Replacing past perfect tense with past tense

For had fallen in a when-clause, can the past perfect tense be replaced with the past tense?

Easily the most boring class was History of Magic, which was the only one taught by a ghost. Professor Binns had been very old indeed when he had fallen asleep in front of the staffroom fire and got up next morning to teach, leaving his body behind him. Binns droned on and on while they scribbled down names and dates, and got Emetic the Evil and Uric the Oddball mixed up.

Solution 1:

If you replace had fallen with fell in the sentence, it is certainly grammatical, and it means essentially the same thing. Rowling has put the whole sentence in into the past perfect to emphasize that this happened a long time before the class that Binns is currently teaching. It may look at first sight as if the last verb (got) is not in past perfect, but in fact it is. There is an elided had before the got (more specifically, the had before the fallen also modifies the got).