1. It's 100 pennies for 1 ad.
2. There's no official place to change them, you'll have to submit a support ticket.
3. If you want to change your Kaho/PCB to another sprite you just need to know what the headgear is called.
4. To change your custom Kaho to a new unimplemented sprite it's 15 ads, it's only 5 ads if someone else has bought that sprite. This is the same regardless of whether the headgear came out recently.
5. There's no ETA on the ad transfer tool, even if there was it wouldn't be reliable.