I concur that new code just needs to be copied and published for me to start working. If new settings/device features are added you may need to re-configure, but not just for new commands or UI designs.
Note: It is possible to utilize the GitHub integration to quickly update code from connected repositories including custom user repositories if their GitHub structure conforms to the ST structure.
SmartApps are a bit different. When updating SmartApp code, you will want/need to open each SmartApp instance and press Done to re-initialize the SmartApp. This is particularly true if the updated code pertains to subscriptions or schedules.