Do you first need to remove the system from the logical system? You may have to remove the entire logical environment from the solution to allow for this to occur.
Below if you find the system and it doesn't match the leading role drop down, you can double click on it and change the value to blank. If it does match, you will need to remove the entire line item.
Will be interested in seeing the answer for this one, I have some systems in the environment that should be upgraded an will have the same results you have run into. 