Upgrade to more one plugin or theme at a time … possible?

Wondering if it’s possible to offer upgrades on a theme by theme / plugin by plugin basis … as opposed to account-wide upgrades.

The account upgrades are great, but it might be more digestible for users to upgrade their account one plugin at a time you know? So instead of paying $20 for an pro account, users could pay $5 for the functionality they want and upgrade to more if they need it. Is this even possible with WordPress?

Likewise for themes — instead of upgrading to a pro account that cracks open a variety of available themes, I’d like to build on a model in which the user commits to one theme when upgrading … thus, enabling the opportunity to make a sale each time they want to change themes.

Couldn’t find anything in the forums on this … open to reading through everything if I’ve missed it.

Cheers