Hi Matt,
Your understanding is correct that “you have to either replace the subscriptions with completely new business premium seats to replace the 20 we already own and migrate the users over. Or alternately I could upgrade them directly through MS but go to month to month model until this first year runs out”.
About the questions “Does anyone know of any benefit of pre-paying through the partner instead of managing it directly through the office365 portal?”, there is no Official article about it. Since the sale policies of partners and online are different and our forum focuses on the online sales, I suggest you contact partners directly. Also other community members are welcome to share it with us in the forum. Your understanding are pretty appreciated.
Thanks,
Jessica Zhang