I'm looking for a mod that improves soulgems so that they will not be wasted where for instance if you have a grand soul gem but not a greater one, capturing a greater soul wastes your grand soul gem. I would like to know if there is a way to assign a newly captured soul to a particular gem, or just to discard it. This would also mean no more petty souls getting stuck in Azura's Star while I'm using Umbra.

The only one I know of is http://www.tesnexus.com/downloads/file.php?id=28504

It's perhaps not entirely what you want, but it does mean that you can empty a soulgem by using it to recharge an enchanted item, allowing you to then refill it with the desired level of soul.?