If I register and actually sign with an unique Apple ID on iTunes and main account, on iPhone, in order to use different accounts between iTunes and main Apple identity (without having iTunes available on my PC), how should I change the account without losing backed-up data?
The description is not very clear, but yes, you can have different Apple IDs for iCloud and for iTunes purchases (if that's what you are asking).
To do this: on the iPhone, go to Settings > iCloud > E*nter the Apple ID you want to use for iCloud Backup* > Turn On "iCloud Backup".
Now, to setup a different Apple ID for iTunes purchases, go to Settings > "App Store" or "iTunes & App Stores" > Enter the other Apple ID to be used for purchases.
That way, you can sport 2 separate Apple IDs: one for iCloud backup and one for iTunes purchases.