Q: Buy app with acct X, then setup acct Y as shared and swithc a device to use acct Y. No you cannot update app because it was "purchased with a different account" even though they are shared

Prior to Family Share we had devices sharing a common account for purchases

We setup Family share an switch the App Store account on some devices to those shared accounts

Now when we try to update existing apps it refuses stating that they were purchased with a different account

Well yes they were, but it is a shared account.

Do we really need to un-install all apps, loosing any data, then reinstall using the shared account ?

If so I think Family share is a non-starter.