Key Assignment in SAP MDG Data Model

Hi,

I had selected Key Assignment for an Entity as "Key Can not be Changed; No internal Key Assignment"  as an option initially and now the requirement changed so i need to change Key Assignment to "Key Can be changed; No internal Key Assignment". 

During change I am getting an error stating: "Data model XX is already active and entity type XXXXX is contained in the active version. In this case, you may no longer change the type of key assignment for entity type XXXXX."

Does any one came across same error or do you know How can change this to meet my requirement?

Thanks,

Sekhar