Assuming your year end is Dec 31, is there any specific reason you can't post the entry on Jan 1?  That will update your Retained Earnings properly for the year going forward and if it is only one entry, you may not need to go through the cost of doing data repair.

If you do wish to do a data repair, you can contact me through my profile here on the forums.  We can connect off forum and with some more information, can determine if I can accomplish what you are looking for.

There used to be a half decent search site for Simply Accounting/Sage 50 consultants but it was discontinued a long time ago and never replaced.