HT204053 I have iTunes ID and Apple ID. Having problems! Can't consolidate into one?

Question

I have read many support postings on this, but none help! Gaaaa!
I have my iTunes/AppStore ID pointed to my Yahoo email.
I later set up an iCloud email account (of course with "@iCloud.com" domain).
When I try to access iCloud email, I get a prompt to set up my iCloud account
on this iPad. I already have it set up! What the heck?
I assume it might be confused because I have two "accounts", but I really have no idea.
Theu are separate functionally and by name.
So I tried to change my ITunes account (used to purchase apps) to use my iCloud email addy.
Apple says I can't use it, because it's an account owned by Apple.
What in the frog is wrong with using an Apple-owned account? That seems best to me.
But I'm afraid that I would lose access to my purchased apps, even tho AppStore should know
(or be notified) that I have just renamed my account, not transferred it to a different device, etc.
I'll bet others have similar problems, but haven't been able to find out how they have solved theirs.
(I also lost six months of Notes a while back, perhaps related to this problem.)

Answer 1

TThanks, randers4, for helping me out.
(I'm re typing this because in switching back and forth between apps
I lost all 8 paragraphs that I typed earlier. And I'm getting
ttwo capital letters at beginning of sentences. Odd.)
I tried your steps, and they worked. Cool. But why hide this functionality?!
Like so many features in iOS, it's not apparent to most users.
I used to write tech docs for HP, so I do understand the RTFM concept. Ha!
THe initial problem arose when I tried to copy some photos
from iPad to OneDrive and Google Drive.
I tried to select multiple files, and then Copy to a diff cloud drive. No dice.
Only options were to Mail, Copy, Print, AirDrop, etc. But no choices for other cloud drives
appeared there, like I expected. (I thought they were there in the past.)
SO I read support forums, and one said to use Cloud Opener. It worked, but
it is so convoluted (in my mind). In Windows, I just multi-select files and
DRag+Drop them where I like. Simple.
AM I missing something, like some metaphor of the OS, that would be
really helpful?
ANd back to an original question--can I merge my iTunes login (Yahoo username) with my
APple ID (iCloud username)?? I want just one password to remember, and change, but
I fear losing all my apps. And I couldn't make the change to an iCloud email addr
because iOS said it couldn't be an Apple-related domain. This seems so arbitrary.
HElp, please?
oh, and iOS asked me to login again, even tho I already was.