You are correct.
But you can use Replace Field Contents with the same calculation as you've defined in your auto-enter expressions to update your existing records. Just do a show all records on a layout where you can get to these fields, then put the cursor into one of them and select Replace field contents. You can also first perform a find, putting = in one of these date fields to find only those records where this field is blank so that any existing data is not affected by this process.
Since Replace field contents can modify 1000's of records in one go and you can't undo it, make a back up copy of your file before trying this so that you can recover from any mistakes.
Phil wins again!
Thanks Phil, once again you saved the day.