AutoCAD Inventor :: Changing IPart In Assembly

First off I am running:

Windows 7 pro - SP 1 (x64)
Processor Intel(R) Core(TM) 2 Duo Pro CPU E7600 @ 3.06 GHz 3.07
GHz
RAM = 8GB
Autodesk Inventor 2012 Ultimate Design Suite

I have an assembly with a couple of parts in. Some of these parts are IParts (NOT Content Centre parts), and they live in my project workspace.

In some case I am able (within the assembly) to right click on the table icon below the part in the assembly tree and change the component to a different configuration. However, I other case I am not able to change.

In the cases where I cannot change, when I right click, IV does not even present the option to "change component".

I am not sure if this will affect the situation or not, but some of the parts were inserted into the assembly as IParts, whereas others were not IParts, but I changes them to IParts after they were inserted into the assembly. I am not sure which ones were inserted as IParts and which weren't, but perhaps this is an issue. i.e. parts that were inserted as IParts originally can be changed, but those that were not IParts when inserted and later converted cannot be changed?