You can't change the format of the CRD constant. What you should do instead is to replac the parameter in your report with a Crystal formula - and determine the format that way. So, for example, if you report is like this:
mydate = {?date}
Then change it to
mydate = Currentdate
There is no parameter anymore and the report will automatically use current date. In Crystal The date format is determined by the settings in the International or Regional Settings control panel.