The zoning change came after I moved in and after I got my chickens. If I had the time, money, and inclination I might have been able to find a lawyer willing to appeal on ex post facto[0] grounds, but it wasn't a battle worth fighting. I had had a year's worth of free eggs by then, and the chickens themselves didn't go to waste.

[0]: https://en.wikipedia.org/wiki/Ex_post_facto_law