Yes, it is possible, but you have to undo the reconciliations back to the one you wish to change. If you are using an older version of Sage 50 you may have to reset the reconciliation module. In either case do a full backup before you start and also have a copy of the reconciliation report showing outstanding items for the month before the one you are correcting.

The full backup is like an umbrella. If you do make a back up, you won't have problems. If you don't make the back up you will most often wish you had.