Greetings Beryl,
You are right about the inability of changing the Tagging Condition; once the tagging process is started, this condition is immutable. You may only influence the Operational Condition of the technical object through switching the status (e.g. ETG > BUG > EUG or ETG > BTUG > ETUG > ETG.)
On your issue, a WCI in the EUG status on a Closed WCD would not cause any conflict for you; Are you certain that the issue isn't in another document?
When you try to switch the "conflicting" WCD, the one in ETG, what is the error? The conflict check should indicate where there is another WCI that prevents you from switching; Frequently it will be another WCD that has had a "green" conflict for the Tagging Condition, but for the Untagging Condition the conflict with the other Tagging Condition is "red".
A solution may be to change the Untagging Condition to be the same as the Tagging Condition and Untag or Temporarily Untag it; As even when you have multiple WCDs, you still only perform the tagging and untagging only once per the technical object. So the WCD which is untagged earlier may need to be untagged _without_ actually changing the current Operational Condition.
What is the message or screen you get when you try to switch your ETG WCD?