To Enable the field on Assign of any entity record, call the below function. On Assign of record, only save event triggered in CRM 2013, so we have to call the function on OnSave. Check the Save Mode for Assign( Save Mode of Assign = 47) and write your logic there..
function EnableFieldOnAssign(prmContext)
{
if (prmContext.getEventArgs().getSaveMode() == 47)
{
Xrm.Page.getControl(“mst_casestatus”).setDisabled(false);
}
}
Hope this helps..