After you hit 200k, any additional credits will go into escrow. So, hypothetical situation, you are sitting at 195k and sell something on the GTN for 8k. As soon as you accept payment, 5k will be added to your balance, putting you at 200k. The remaining 3k would be added to escrow.

Using CC, you can unlock fixed amounts of credits from your escrow, but I don't think it is worth it. I would rather pay attention to my balance and send credits to alts via the GTN at a 6% fee.

If you become a subscriber, you get all of your credits from escrow instantly. If you become a preferred player, I am not sure if you get your escrow back up until the new cap of 350k, or if your cap is simply raised and you still will have "3k credits" in escrow. However, considering that the expansion costs $20 but is free with a $15 one month sub, plus you get 500 CC, I would just sub rather than make a $5 purchase for preferred status.