You dont need to update your whole farm. I would go R2 to XA6 - it will be a new farm, which is okay, you want it separate so you can run long term. Use 1 licnese server so users can launch apps on your current farm and the xa6 farm and only consume 1 license. Within web interface and pnagent, add the xa6 farm so users see the new apps. You are right, mounting the iso should work fine. I would get the new app streaming profile tool, its on the xa6 iso and isolate office 2010 - it works very well - rather than installing 2010 on the R2 server.
http://support.citrix.com/article/CTX124565