get_field will not behave the same way. For a date field ACF expects to find a value that is “YYYYMMDD”. Well honestly I don’t know if ACF will convert “YYYY” properly, but my guess in that it won’t.
There is not easy way to convert from a number field to a date field.
As I’ve posted in the past, the best way to deal with this is to add a new field and leave the old field alone. Use an acf/prepare_field filter to remove the old field. To deal with old posts with the old field in your template code look for a value in the new field and if it does not have a value then get the value from the old field.
