Once they get too big GET RID OF THEM. Do NOT keep them "just in case" because then you are enabling that "just in case" to happen. This way you don't have anything to fall back into, and that is a good thing.
I got rid of them ALL once I grew too small. I sold alot on ebay and made some $$ back to buy new clothes and also donated alot to charity.
But--- I did keep one pair of "fat pants" at my highest size and a shirt at my highest size so I can always check and see how far I have come.