While I think you would be better off creating a numeric variable instead of changing the character variable, that's a different story. Keeping my opinion out of it, here is a possibility:
data want;
length datevar $ 10;
set have;
datevar = put (input(datevar, mmddyy8.), mmddyys10.);
run;
The "s" in the middle of the new format name indicates that slashes should be used as separators.
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