You can change your main at any time. I might be a Takeda main now, but if I play around with him and realize he's not for me, I'll drop him like a bad habit.Lol I'm not a tier whore... :/
But you don't know if you like how the character feels when you play them or anything. I have like a list of characters who I want to try first before any other characters based on their move set and what looks awesome, it doesn't mean I want to main them. That's just, ludacris to me! I don't understand you folks!