OpManager - Cannot change Vendor field

I am working with an existing OpManager 7.x environment. I am setting up the partition % free space on 100+ servers and about 40 of them did not have a 'vendor' specified even though the OS in many cases is indicated. I cannot add more counters until the vendor and OS are specified. OpManager however does not appear to let me change the vendor to anything other than 'unknown'. I would like to just change it to 'Microsoft', I also tried Dell/HP in some cases but get the same result. When I do change it to Microsoft by clicking the Vendor link or the Edit button above the screen refreshes and the result at the top of the screen says "Field values are successfully updated to the database" but the vendor remains "Unknown" and I cannot add more monitor counters.

How can I go about changing the Vendor field? Is there another method or a way to force it?

Thanks in advance.

John