Quote from rmorse:
If your trading in a Portfolio Margin account, and your equity drops below the $100K threshold, you would be expected to add funds to your account to maintain the PM margin and risk profile associated with the account. If you don't add funds the next day, the following day, your account would become Reg-T. If your account does not use more than the requirements of Reg-T, you would not have to put in more money. The account would now be a reg-T account. If the account under Reg-T has a margin call, you would have to satisfy that.
I guess some Prime Brokers can be stricter. However, I think the procedure above will be the most common. Open or close does not come into play.
This is my understanding as well.