I am relatively sure you can change guilds on a whim, but may require a log-out to do so. No cost. I'll have to double check the guild tab on monday as I was randomly invited to other guilds in spawn areas. I'm sure there will be people switching specifically for boosts, but that also means you're adding influence only to that guild, increasing their rate of boosts. Sticking with one guild, supporting it with influence (even if you may not be 100% efficient at the start for small guilds) is really the way to go.