alwalaska, I'm not an expert at all either. This is the first time I've done this.
re your first question: I think they just randomly assign a cabin. I spoke to my PCC when I booked the inside cabin IA, told her my plans to upgrade with BofA points to a Balcony BA. She told me that after the upgrade was complete, if I wanted to change the rooms (within the same BA category), I could call her and she could do a cabin change.
re your second question: Not sure what you want to test, but you purchase the upgrade like any online purchase: put it in your 'cart' and then check out. There are lots of warnings that the transaction is not refundable.