OK, so say you have 1000 existing records. The form rule will work on a going forward basis for just 1 of those 1000 records because that is the only one has old value populated.
So, the question is how to populate old value in the other 999 records.
If you have less than say 500 records, then you could run a report and get those two columns side by side and Grid Edit and highlight the source cells and then right click to copy and then paste.
But if you have a lot of records, then another method is do this just once.
Change the field type of old value to be a formula numeric.
Make the formula just be
[Actual Est Date (concept)]
Then change the field type for old back to to be a numeric field type. Quick Base will drop the calculated value that was there by calculation into each row of data. So even if you had 100,000 Records, that would happen basically instantly.
Now your form rule will work on all your records and not just that one where you hand edited.
