I know that when you purchase a supporter rank for $10, it will only cost another $10 for VIP. But is this still that case when the first rank it gifted? I'm kind of unsure about this. If I was to upgrade will it be as simple as just buying VIP?
When you're gifted a rank, that is now you're rank regardless of how you got it, so if another $10 is stacked on top it will, in this case, bump you into VIP so to answer your question yes it is that simple.