14 DAY TRIAL // Ubuntu 15.04 (Vivid Vervet) will arrive in just a couple of weeks, but if you really want to, you can upgrade your current Ubuntu installation right now. You don't have to wait until launch day, but there are some small risks.
Ubuntu 15.04 is not considered a stable release yet, and developers are still fixing various issues. It's possible to upgrade right now, and it's quite possible that you won't even feel that you're running an unstable release. On the other hand, there are some risks involved. For example, if you have any third-party PPA repos, they will be disabled, and those devs might not have updated for the latest Ubuntu release.
Albeit small, the possibility of breaking the system exists. On the other hand, the upgrade can break your system even after the official launch, so it's a tradeoff. It's your decision if you want to upgrade to Ubuntu 15.04 now, and this is how you do it.
How to upgrade to Ubuntu 15.04
If the user wants to upgrade to a stable version of Ubuntu, the process is rather simple. You just open the Software & Updates tool, go to the Updates tab and change the option at the bottom to suit your needs. If you have Ubuntu 14.04, for example, you might now want to get informed about new releases unless it's another LTS. Also, you can change to the "any new version" option and upgrade from an LTS version to a non-LTS version.
The update path is usually unlocked until the launch day, so it's a little bit tricky to start the process, but this is Linux, and very few things are not possible. You will have to open a terminal and enter a single command (root is needed):
Enjoy!