Yes, the witnesses have control over all aspects of Steem, the blockchain/database that Steemit runs on. You'll occasionally hear the term "hard fork," which (in Steem) means that there was an update to the code for Steem, and the witnesses got together and agreed that the new code should be adopted. They have done this in the past to change the reward structure.
I believe for the update to take place you need 75% of the active witnesses to sign on to it.
