Well... Let's look at this from different way(s) then...

"Margin = 0 just means that no new trades are opened" is not a correct statement - I had Margin=0 and a new trade was opened.

In the manual link above it is stated "For investing a certain amount of your account balance, use Margin". This explicitly says that Margin is relative to the account balance - i.e., Real $$$ => Real trades. It also states the same as you said: "If Margin is 0 or negative, no trades are opened". Together with the above statement that Margin is relative to account balance, Margin=0 means no (new) Real trades are entered.

Re. the last paragraph, I don't want to close trades via Panic - I simply want to do what the manual says to do for an upgrade when trades can't be resumed - set Margin=0 and let the existing trades be played out by the old version without entering new trades and without using additional margin in the account. Any new trades and new margin should come solely from the new version.

Is that clearer?

Thanks.