I have an alpha version of Ubuntu.
When the stable version is released do I have to upgrade or is it already stable from the installed updates?
The normal sudo apt-get update and sudo apt-get upgrade will get new repo lists and packages as they are released and ultimately, you will have the same packages installed as the final release, including the various bugfixes as the version transitions from -
Alpha -> Beta -> Release Candidate -> Stable Release
You do not need to 'fresh install' the stable version later.
For Ubuntu 15.04 (Vivid Vervet), the release schedule is -
