I believe you may be confusing effective date and date of report. You can make a revision/correction at any time after the original report was signed, and changing the report date to the date of the change indicates to the reader that a change was made. However, if you want to update the report with new market data, then you would need to have a new effective date, and thus a new report date. But you would need to re-affirm that no alterations have been to the observations you made as of the prior effective date. Then disclose the prior services. Also, if this is for a FNMA lender, then you also need to re-visit and re-observe.