I already talked to Sarah about this - I wouldn't change something on her site without her approval - and she agreed that it would be easier than the stat system for everybody on the site, and a lot less hassle having to always constantly changing numbers.
With people that have already been rping without tiers, firstly their applications will be looked at to see what rank they are currently so that they can have a base power level, and then if they have upgradeed their power through modifications to their abilities through training then that power level will be changed accordingly.