You can uninstall the old ones without issue, but you don't have to...
The major releases, ie., Fab 2.xxx - 3.xxx -8.xxxx are independent of one another and installed in separate program folders.
Even if you choose to uninstall, many of us keep older installers...just in case
On a rare occasion, an older version will be successful with a dvd where the current versions may stumble.
Yes, any new version replaces the earlier version.
For up-to-date decrypting and new features, it's recommended that you keep current.
Right now, that would be v.8073