Imported furniture, textures and plug-ins are not removed when you install a new version and when you uninstall Sweet Home 3D (if you want to delete them, see How can I uninstall Sweet Home 3D? in the FAQ).
In the case of the portable version, imported stuff won't move automatically and you may have to copy or update some data as explained in the README.TXT of the portable version.
In the case you are under Mac OS X and moved from the free version to the Mac App Store version, the folder where imported stuff is stored may have to be copied.

In case you need to move the imported stuff to an other computer (or backup it), read also this thread .