Transferring app account ownership
Like it's stated in the linked question, this isn't possible... As a matter of fact, a lot of users are suffering from this issue and some of them even have contacted Apple about it and here is what Apple said: 


  ... they told me to reset my password so that both userids have the same
  password. When you get notification that an app is to be upgraded, you
  will get a prompt to enter your password. The prompt will indicate
  which userid you bought the app under. I have not had to logon to
  either id's to see which apps are updated. But you do have to log off
  and logon in iTunes to see the apps you purchased under that specific
  userid.


I have multiple accounts myself (one for the US App Store and one for my own country) and I switch accounts in the Settings &lt; Store menu in order to get to donwload/update apps from the different stores.
In my opinion, that's the only way to get/update apps bought with different accounts.