Change Apple ID

I just turned on Mail in iCloud. I am looking at switching from Gmail to iCloud for my personal email. My Gmail address is my current Apple ID & the email address I use to sign in to iCloud. When I try to change my Apple ID from Gmail to iCloud, I get an error that if I already have an iCloud account, I can just use that email address, but it won't let me delete the Gmail address so just my iCloud address is associated with Apple.

How do I go about changing my Apple ID, while still being able to download all of the apps that I have previously downloaded under my Gmail address?