Some may consider this a faux pas.
It can also be confusing. I know that some people in their reviews have a “second opinion” type section, where they reference the ratings/reviews of others. If you get linked to that when you rated it one way, and then change it, it’s awkward.
But opinions change over time.
Other times, I just feel like I’ve explored a genre more thoroughly and something that I found average at one point might feel more mediocre than anything now. I’m thinking mostly of the dystopias, in this case. I must admit, I’m a bit over the genre, and looking back on my reviews of Taken, Dualed, and Across the Universe, and considering how apathetic I feel about them now, I only felt right in letting my ratings reflect that.
Sometimes this comes not just after looking through my reviews, but rereading the books. That would make the rating shift more legitimate, I do admit – but I don’t have time to reread everything. I recently reread Shut Out and it carried so much more significance to me the second time through because of its meaningful conversation about sexuality and slut shaming. Its feminist themes appealed to me so much more, and the romance was just as feelsy as the first time, so I boosted the rating.
So yeah, I may change my ratings over time.
I will admit though that this is most often a shift downward. I can only think of two books where I moved a rating up: Shut Out and Parallel. In those cases, the books just lingered, actually gaining emotional significance over time. But after time has passed and my opinion has settled, I feel like having that 0.5 or 1 orange shift upward or downward is just me being honest (especially when the wording of the review still fits) – and I only ever shift a rating once per book. See, if I refer to books over time, like in Top Ten Tuesday lists or whatever, people might go read those older reviews, and I still want them to reflect my honest opinion.
And I must admit, this is also a product of my short but accelerated reading journey. Having read so much over the past two and a half years, compared to almost NOTHING before then, I’m learning as I go and becoming more objective. The ratings in my first year were extremely generous. My ratings nowadays are more objective and thought through, so I don’t expect that this will happen too much in the future.