Not had this with dates but had the same issue with numbers. Some numbers in a column came across as numbers and some as text and the format option wouldn’t changed the text cells back to numbers.
Got round it by using the left and right text functions to split the cell contents up then the concatenate function to “reassemble” the numbers stored as text. After that was able to convert to numbers.
So I would try changing the format of all dates to General (which should display as a number)then convert them all back to the date format you want or try multiplying every cell by 1 to convert to a number then changing to the date format you want.