Plan upgrade

Hello
I am using your plugin on one of my site, Want to know about a feature of membership upgrade. I have 3 plan, Free, Gold with subscription rate as $100 and Platimium with subscription rate as $200, I want ot give a facility where people can upgrade to platimium from gold by paying only $100, is it possible in the plugin? if yes then how?

thanks in advance

http://wordpress.org/plugins/paid-memberships-pro/