It will help alot so say you have Elite in Zombies and you want to upgrade to Sponsor you can through Rank Upgrades. Please add it <3 it will be alot of help to me and others so we dont have to buy the next rank for alot more when their could be Rank Upgrades.
From my understanding, the AEM shop actually considers your rank and automatically removes the cost of said rank from the price of the next rank. I could be wrong but it's definitely more of a @Sway and @Calcium solution (as they are the only ones who can handle buycraft)
What mew has said is correct, I have ultimate on survival but have ranked up from VIP to Elite to Prem to ultimate, each time I have received the discount of the current cost of my current rank at the time. Now that I plan to go to extreme when i check the shop it will give me $100 off extreme (which is $200) as shown by this screenshot https://imgur.com/a/I0mAF