Trial Version Installation

I work in IT and I frequently re-image tablet based computers for our users.
Our users all have their own, individual licenses of Microsoft Office
(various versions - most of them OLD), and install them onto their tablet
computers once they get them back.

We want to encourage our users to upgrade to the latest version by having
the trial version installed on the computer when they get it back.

To streamline the process, I was hoping that there was a trial key that can
be installed onto the image so that I do not have to download and install the
software each time I re-image a unit.