This assumes that all dates are after 1/1/2000 - you could change to a conditional inclusion of "19" or "20" based on the year range, but this is simpler (for now, at least) I have assumed that the dates start in row 2 and that there are no blank cells or
cells without a five or six digit value.

Sub RedoDateStrings()

Dim rngDate As Range

Dim rngDates As Range

Set rngDates = Range(Range("B2"), Cells(Rows.Count, "B").End(xlUp))

rngDates.NumberFormat = "@"

For Each rngDate In rngDates

If Len(rngDate.Value) = 5 Then

rngDate.Value = "'0" & Left(rngDate.Value, 3) & "20" & Right(rngDate.Value, 2)

Else

rngDate.Value = Left(rngDate.Value, 4) & "20" & Right(rngDate.Value, 2)

End If

Next rngDate

End Sub