No, certainly not!
The worst case scenario is that the installation fails (not likely!) and SEM lets spam through to the users. In other words, the users receive the spam they are used to receiving. On top of that, SEM is very easy to uninstall completely, but our guess is that you wont want to after trying it out.
Bottom line, you can safely install the free trial and test it as much as you want without worrying about down time or other complications.