Procedure for reducing capital?

If I wish to reduce the capital invested in a Z system, say by 50%, what is the proper procedure? I can Test to find the new Margin setting appropriate to 50% capital, but then what? Change to the new Margin setting and wait for all old trades to complete before withdrawing the 50%?

Thanks.

Yes. As long as old trades are still open, you still need the original capital on the account.