How to get preOwner and postOwner in Plugin

SecurityPrincipal assignee = (SecurityPrincipal)context.InputParameters.Properties["Assignee"];

Guid guidNewOwner = assignee.PrincipalId;

Moniker moniker = (Moniker)context.InputParameters.Properties["Target"];

Guid id = moniker.Id;

//pass this id and get owner id from lead

ICrmService service = context.CreateCrmService(true);

Guid guidOldOwner = GetOwner(id.ToString(), service);