Currently, there is no provision to merge accounts which would allow you to combine everything and get rid of the unwanted account cleanly. I've not heard of a process to delete one other than letting it sit unused until the next time the service prunes unused accounts. That would at least let you walk away from everything related to the old account and perhaps enable it to be pruned / reused sooner rather than later.
In the past, unused accounts have not been deleted and freed, but with the move to iCloud some older accounts could get deleted and freed up but only Apple knows if this is planned since nothing has been spoken publicly about this.
What you can do is change all personal information on the account you wish to leave behind and set the password to something written down in a safe location and wait for it to expire.
You can change your Apple ID on all accounts that are not ended with @mac.com or @me.com which is a welcome change to the past when you couldn't even do that. As it's not perfectly clear what your end goal is, why not reach out to Apple's billing and account support and discuss your particular needs with an actual employee trained in how the system works to see if they have a better answer for you.