I'm not familiar with either of these franchises, so I can't answer the question myself, but here's how I see it:
If the two franchises are similar enough that someone else is likely to make the same mistake as this OP did, it makes sense to leave the tags as they are, or at the most, add ASoFaI tags.
If there is little or no possibility that someone else will make the same mistake as the OP did, then it would be acceptable to change the tags, and probably even remove any references to Pratchett/Discworld/any titles, locations, characters, etc, associated with this franchise.
It probably isn't that important either way. Story ID questions are often duplicated, and site policy is to leave the new question open until/unless both the new question and the old question have accepted answers. The prevalence of Story ID dupes suggests that previous questions about the same story are rarely discovered by the person posting the new question before said question is posted. In other words, it is unlikely that someone who has a Story ID question about ASoFaI will find this question before he or she posts the new question, and it won't make any difference whether we change the tags or not.
As such, we can change the tags if we think that it might be beneficial to do so, or we can leave the tags as is if we think that it might be beneficial to do so. But whatever we do, it probably won't matter anyway, so if we simply ignore it altogether, it probably won't make a difference.