Hi Emma,
What about something more basic to fix the issue. You could have the following:
1. Estimated Revenue - This could be the main field used for the Estimated Revenue throughout the opportunity process. This field would be used until the Opportunity Business Process Flow is completed
2. Actual Revenue - When this value is populated setup a real time workflow that will take the value of the field of the Estimated Revenue and populate another field called "Original Estimated Revenue" and take the value of the Actual Revenue and populate the Estimated Revenue Field. You could also do this second part through a business rule as well.
This will allow the current "Estimated Revenue" to populate the "Actual Revenue" on the Opportunity Close process and provide you the value you expect. You would also still have the Original Estimated Revenue for any reporting of Estimated to Actual.
Just a thought on a different approach if you want to fill in the Actual Revenue before closing the Opportunity as won...