Hi Philip,
To change the property (event the ReadOnly property) of a ReadOnly cell, first the IgnoreReadOnly property should be set to true and then the ReadOnly cell’s property can be set/changed. For the mentioned scenario what you can do is, first set the IgnoreReadOnly to true and then set the ReadOnly property of the particular cell to false/true and again set back the IgnoreReadOnly property to false.
Please refer to the following link to know more on setting properties for the ReadOnly cells.
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=338
Let us know if you need any further assistance.
Regards,
Calvin.