I'm curious to your use case around this? It seems odd to need to adjust it on the fly while a topology is running, or I've misunderstood you!
If you store your consumer state in Zookeeper, you CAN adjust it between topology deploys by manually modifying the stored state, and I've done this to deal w/ maintenance or service issues to roll back to a specific point in time. Unsure if you're able to do this when consumer state is stored within Kafka itself.
As a side note, I've been toying with a Kafka spout implementation that allows dynamically consuming arbitrary ranges from topics that is to be open sourced here soon.