If you have MX 2004, you really dont NEED to switch. Flash 8 helps with better looking effects using its filters, some new action script, and it can handle lag much better. And no, if you install the trial, it will not overwrite your previos program. But be sure to save everything in MX 2004 format before the trial ends or you will be left with a bunch of files that you cant use because you dont have 8 anymore.