I am currently on the 4.5G for $40, with Refer-a-Friend rewards and Autopay reward.
I would like to change my plan to the 500 MB bonus data for Autopay promotion.
Will I lose the Refer-a-Friend rewards when I make the change?
Yes you will keep the friend reward (and autopay / loyalty / community)
No you will not lose the friend reward (or autopay / loyalty / community)
Every single plan you can currently select is eligible for rewards.
The only way you will lose your friend reward is if they do not pay their bill. Once they start paying their bill again the reward will come back.