You can individually downgrade or upgrade any of the individual packages which make up your bundle.
Call in to 150 (from a VM phone) or 0345 454 1111 (from any other lines.) Enter the main menu. On a Sunday go with option 4 -> option 2. M-Sa wise, go with Customer relations: option 4 -> option 5 (M-F Customer Relations is staffed between 8AM to 8PM. 8AM to 6PM on a Saturday. Yes downgrades need to be called in about as My VM only offers upgrade options.
Note: downgrading can recontract you. You would be of advised of any recontracting by the agent you speak to, before you go ahead with the change. Also, changing up your bundle deal can cause you to lose the discount that is applied to the bundle you signed up to. So it could end up being more expensive and recontract you. All of this you should discuss when calling in about it.