:o I didn't know about v1.5 :) You probably mean v1.4.
Upgrading will overwrite all files so it will be a bit tricky if you do not remember the files you've modified. Apart from that, upgrading is as easy as its written in http://docs.tastyigniter.com/upgrade-guide/
After upgrade your existing data wont change, but make sure you back up your default mail templates.
