[aephi alum]

^ What she said. If it's an NPC sorority and there's a chapter at your new school, you can try to affiliate with that chapter and continue to enjoy the collegiate experience. If not, you become an alumna. The alumna experience is very different, but can be very rewarding. But if you outright disaffiliate and turn in your pin, you will lose out on that experience. If you can't affiliate with a chapter at your new school, you have nothing to lose by going alum rather than disaffiliating.

If it's not an NPC sorority, you'll have to check with your sisters.

Either way, I'd tell your big sister before the end of the semester, and let her guide you. The recruitment chair needs to know you won't be back next fall to help with recruitment (if you have fall recruitment). The treasurer needs to know, for budgeting reasons, that you won't be paying dues after this semester. And you don't want to burn any bridges with any of your sisters, since you might very well run into some of your chapter sisters at conventions or find yourself working with them during your involvement on the alum level.