I do not see any major problems that would stop you from upgrading from 10g to 11g.
The underlying application server changes from OAS to Weblogic but apart from that you will not see any changes. Certainly Plus and Viewer operate the same way as does Admin and Desktop. Even the EUL iitself is 100% compatible between the two. Mind you I would recommend using an 11g database.
The tricky part is understanding how to install Discoverer 11g and then having someone who can learn how to manage Discoverer using Weblogic.
From an end user perspective everything will be fine. In fact, in my opinion Discoverer 11g runs faster than 10g, presumably because Weblogic handles the web stuff better than the old OAS.