They won't disappear, but they will block the terminal from updating. The terminal must be totally, completely, empty before it will update to a new expedition (once the next one is released). You could leave stuff in the terminal, and then once you see the announcement of a new expedition, load everything into your suit, save by jumping into & out of your ship, exit & reload. That should update the terminal, and then you can load everything from your suit back in & start the expedition. Doing it that way will save you some storage space elsewhere, and make it harder for you to accidentally use/spend/sell stuff that you plan to take into the next expedition, but it is also more of a hassle once a new expedition comes out.