When the latest rewards came out, Blizzard said
"If you still have active recruits on the previous Recruit A Friend reward track, you will continue to earn those rewards as long as your recruits stay subscribed. If for any reason, your recruits lapse for three days or more and then return, they will begin awarding credit for the current reward track.
Any recruitments made after March 21 will be assigned to the new rewards track; Recruiters are able to gain rewards from both tracks simultaneously."
When the rewards change again, it’d probably be the same, but Blizzard could always change how it works.