I said I would pick up the cake at 8: 30 in the evening after 9: 00 in the morning. Now I want to pick it up at 8 o'clock or earlier, okay?

You are only half an hour early, so it's not a big problem. Generally speaking, they will finish ahead of schedule. It's impossible to wait until after eight o'clock, but the later you do it, the fresher it is. Some specialty stores will pinch this time to make it exactly at your appointed time. If you want to call in advance, it should be no problem!