You can delete the old eval license and place there new eval license.
The easiest way I use is to delete the file from the license directory (I hope I remember it well: /usr/local/Extreme Networks/Netsight/license/)
if there are more licenses there is license hierarchy = eval does have the highest precedense, the next is ADV the next is full the last is BASE.
Regards
Zdeněk Pala