I am unable to reproduce this with my Windows 7 virtual Machine using the 4.4-GA RC3 installer. Is there any more detail surrounding what you were doing when you experienced the problem?
I Created a new Instaview project using the Apache Weblogs Sample. ran all the way through the set up and data loading. Then I clicked on Edit in the Model processing panel. The modeler opened up, I changed the aggregation of 2 different measures from SUM to Average & Minimum and saved the model. I went back to the Instaview perspective, created a new analyzer report so it would know about the new model and was able to use those fields with their new aggregation values.
did you follow a similar pattern? Are you using the the RC3 installer? did you use the PDI installer of the BA-Server installer?