Yes, you should be able to use an Update query to do what you want (provided
the field in question hasn't been set to Required)
BTW, take a look at the Format property (or function). It actually allows
you to specify up to 4 different format strings for a particular field: one
for positive values, one for negative values, one for zeroes and one for
Null values. That means that you could use a format like $#,##0;($#,##0);""
and have blank show for zero values.
?Format(23, "$#,##0;($#,##0);""""")
$23
?Format(-50, "$#,##0;($#,##0);""""")
($50)
?Format(0, "$#,##0;($#,##0);""""")
?Format(Null, "$#,##0;($#,##0);""""")