I do not really understand why you get this error other than if it is a new record in a main table that has not been written to the table yet and the memo is in a different table.
You can get access to assign the current record number to a variable and close the form, reopen the form and move to the same record. But you should be able to achieve the the same outcome by moving to the previous record and then to the next record thereby ending up on the same record in the current session. Or you can do it using a record set clone to update the memo field, but the current record must have been saved by access.