This is actually a very good question. I will allow you to change tiers only if it is for a higher tier, like in your case how you are wanting to go from the beginner to amateur tier, but you will be locked into the higher tier for the remainder of the month once you make that change. My goal was to make this as fair as possible and I appreciate that you know that your street photography will be above the beginner level.
I will also update the rules to reflect the new changes as well.
Hahh well thanks for you answer but in the meantime I've realized that my pictures are 10 times worse than I've remembered them hahaha :D so I'm staying in beginner league :D