I have only just found out about a new feature in 3.6 version.
The ability for a customer to upgrade or downgrade their account at user group level.
This can be set for admin only or actually allowing the customer to do this via the user home page.
This is great if you have retail prices and also trade prices + with subscription levels added on this give great control over pricing etc without much admin intervention.
One thing missing though is the ability to set the minimum order at user level (this can only be done at site level)
I have asked viart for a quote and they state it will cost $100 to be able to set a minimum order per order at user level.