First make a backup of the file, in case playing with date formats gives you 3548910200 instead of a date!
Then check that Excel is recognizing it as a date format. I tried entering 1945-05-14 in a cell and it automatically converted it to 05/14/1945 for me. Try clicking on the cell, going to format, and picking the date format you want.
If that *works* you can just select all of the cells and change the format in one fell swoop.