Nostalgia for fond memories commonly produces positive emotion (for further review, see Zhou, Wildschut, Sedikides, Chen, & Vingerhoets, 2012). As for the increase in your appreciation of these particular puns, I've mentioned in a very recent answer of mine to another very recent question about humor that positive emotion alone may encourage the sense that something is funny. Depending on the nature of the particular puns, they might also appeal to a certain sense of humor you've had to develop with age and experience.
This last point is just conditional speculation though; I also admit that the article I've cited doesn't pertain to humor directly, and I can't provide more direct evidence of the sort your question requests offhand. I hope someone else can!
Zhou, X., Wildschut, T., Sedikides, C., Chen, X., & Vingerhoets, A. J. (2012). Heartwarming memories: Nostalgia maintains physiological comfort. Emotion, 12(4), 678–685. Available online, URL: http://www.wildschut.me/Tim_Wildschut/home_files/emo-2012-05385-001.pdf.