what plugin step should be added on Re-Assigning owner? CRM 2011

Hi,

i have a requirement where i need to update the Associated record owner on Re-assigning the account to some one.

I have created a Step on Pre-Operation for Update account and debugging below code snippet which is written in plugin

var context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));
            if (context.InputParameters.Contains("ownerid"))

But the IF statement is false always.

Then i have changed  step to Pre-Operation for Assign account , Still it doesn't contain the ownerid in Input parameters bag.

but nothing helped. Could any body help me on this.

Sreeni Pavalla