Development of Creative Commons licenses

[James Byers <jbyers AT jbyers.com>]

Hello,

I help run http://wikispaces.org, a free wiki host whose 
member-contributed content is currently licensed under by-sa 2.0.  I'm 
hoping someone can help clarify the process of moving to by-sa 2.5 for 
existing and future content.  Every page on Wikispaces has both a link 
to the by-sa 2.0 license and the corresponding RDF block.

Is it correct and appropriate to simply update these to version 2.5 or 
should existing content keep the 2.0 versions?  What other issues should 
we consider so that we're not adversely affecting the rights of our 
member-authors in the process?

Thanks in advance,
James