Discounts would depend on what you are dropping.
But generally you can downgrade your service (ie go to lower tier TV package or Internet) without affecting contract.
Discounts may be affected.
If you log in to Verizon site and go to change services, you should see how things change if you change services.
Take note, besides what CRobGauth has mentioned, upgrading from a package such as 75Mbps to 150Mbps may occasionally have a charge associated with it for service upgrades. These seem to occur when you have to either purchase a new FiOS router, or if Verizon has to dispatch a technician to replace your ONT due to your ONT being on BPON.