Del is correct about the not being able to save as feature.
However, it is my understanding there is some built in compatibility. My understanding (Del or someone please correct me if this is incorrect), is that reports created in CRXI can be opened in versions as early as 9 and CR2008 in version 10. What I believe you cannot do is readily edit the later version in the earlier version. So it may depend on what you want to do with the earlier version.