The Basic Membership would just kick in after the Upgraded expired. When it expires, you'll only lose things if they're above the limit for the membership level you would move to, like if you purchased a Basic, if you had anything above 50 (the newest one above that number) would be locked and put into the recycle bin and need to be cleared or purchase another Upgraded to unlock them.

Someone in RAOK more than likely put in for you to have the Upgraded membership but the names stay within the group. I would suggest putting something like "Thx for Upgrade" or some variation in your handle if you want to say thank you, but RAOK is all about being kind to those deserving and seeing the gift appreciated and used is a great way to say thanks in its own way.

Hope that helps!