See this support document. Change your Apple ID - Apple Support
Excerpt:
"If your email address ends with @icloud.com, @me.com, or @mac.com, it's already an Apple ID. "
This is why you cannot change the email address for your current Apple ID to the iCloud ID, because it is an Apple ID.
You can also take a look at a couple of other support documents that might help you. As Gail told you earlier, you can use the iCloud email ID for the forum if you like, rather than your other, since you have not accumulated many points it won't matter. Frequently asked questions about Apple ID - Apple Support
Create and start using an Apple ID - Apple Support
Set up an Apple ID in iTunes - Apple Support