PROCEED WITH CAUTION.
If you install an upgrade or downgrade, it will replace your existing rom. If you are using a stock version of your device-specific rom, you can do it with ODIN. You should back up your contact, calendar, texts, sms, pictures, etc. Contacts and calendar are on google if you set it up previously. Most apps from Play Store are stored in my apps in Play Store. Text, sms, and picture on your device will be permanently gone if you don't back them up.
If you are using a custom rom, you must be sure that it is specific to your device. Flashing the wrong rom can brick your device. So proceed with caution and read the forum for that particular rom carefully to be sure that all the features you want are working. Some roms have some bugs such as decreased battery life, camera features, etc. I have tried flashing many roms with success and you can do it too. Also keep in mind that your warranty will be voided if you have issues and your carrier knows your device is modded in any way. Best advice I can give you is, if you are not sure and not willing to take a risk, stay stock. This is not to discourage you; just proceed after you feel confident enough to take the leap.
Disclaimer- Again, I am NOT an expert, so if I am wrong, someone correct me. What you choose to do with your device is your decision. I assume no responsibility for what you choose to do.