The consumer preview is way outdated. Even if you were to install it or the more current Release Preview, you wouldn't be able to directly upgrade to full Windows 8... you would have to do a reinstall.
With that being the case, your best bet for a trial is the actual Windows 8 90 day evaluation that Microsoft themselves offers. You can download that here:
http
/msdn.microsoft.com/en-us/evalcenter/jj554510.aspx
You cannot upgrade the 90 day evaluation to a full paid version of Windows 8, either, so you'd still be doing a reinstall... but that is the path I would take if I were you. I would install it in a Virtual Machine though,. Something like VMWare Player would do, before I gave up my installed programs, especially if I were just going to trial it.