Indeed, the difference between 15 and 51 is larger than a mere transposition of digits, with an almost inevitable impact on re-reads.
I've had a similar experience of change, but in a positive way, with some of Ngaio Marsh's books. As a teen, I loved her books for the way she used them to share her primary passion, theatre, especially Shakepeare. But her attitude toward the land of her birth and its indigenous people (from whose language her own name comes) seemed ambivalent at best. Having re-read 10 or so last year, I was intrigued to note that her perspective on these islands and its peoples was much more nuanced and thoughtful than I'd previously noticed. Certainly her views on people genetically blessed with an abundance of melanin are far ahead of those repeatedly expressed by Allingham (whose writings in this specific regard remind me ever more of Enid Blyton)