You would need to open a sales ticket and discuss this with them, however we don't typically do trial accounts. Also, semi-dedicated accounts are placed on a specific group of servers so it would require transfering your website to the new server. With the potential for DNS propagation delay, I wouldn't suggest a short term transfer.
I'm a happy customer and from my experience, MDDHosting shared is already faster than any other shared hosting I've tried. Here's my review back in Feb 2012: http://forums.mddhos...lmost-one-year/
I really want to try semi-dedicated to see if my heavy websites improve but I don't want to do it if it's too complicated or if it messes up my site.