I'm trying to make a multi-guild bot and I'm struggling with making a command that changes the bot avatar only for the guild where the command was executed!
I tried to make a DISCORD API call to change the bot user avatar but I ended by changing the avatar for all guilds.
You simply can't do that. Bots don't have access to some user functionalities like blocking someone, joining a server without a moderator's will and, in this case, change it's avatar/username depending on a server. An alternative is to create a webhook. You can change (with the necessary permissions) it's avatar and username at free will and it can send messages, embeds, files etc.. there's a full guide on discord.js webhooks if you want.