Email workflow when value changes - can't compare PRE to post value

Hello, I am trying to create a workflow that notify managers when their sales reps (SR) have updated an opportunity. For example, the manager needs to know when the 'SR Forecast' field has been updated. I have tried something like this: [<plSR_Forecast_ITAG>] <> PRE ('<plSR_Forecast_ITAG>'). But it appears that I cannot use the PRE() function when using a Trigger Event of 'When modified record saved'. It appears that the previous (PRE) value is no longer available for the comparison. I included the PRE and post actual field values on the email, and triggered the workflow to create the email, and, the post value is provided in the