Fans are conservative by default. That true in about all fandoms because it’s in a definition of a fan to love how something is now.
When you introduce changes, it is only logical that fans will be the ones noticing these changes first and resist it because it changes their relationship to the product as fans. This resistance can lead to protests and boycotts but more often than not, just nerd-fueled rage.
TL;DR: If you weren’t conservative to some degrees, you wouldn’t be a fan. Fans have different levels of tolerance to change but if you change something you loved, you might not love it anymore.