Hey Lacey,
If the field becomes visible to the sync user, this will not by itself cause all of the values in the field to be synched down and cause a backlog. These values will only be synched when there are updates to the lead record, so you can do a mass update through the data loader on your records for those that have a non-empty value in this field, which will cause all of the records to have their values synched to Marketo. Depending on the number of records which will be synched, this may cause a backlog, but it is dependent on the number of records which need to be updated.