How do I change the underlying field definition?
What is an Integer Format?
I believe you are confusing a field's format with it's datatype.
Open the Table in design view.
In the upper panel, look at the field's datatype property.
It should read either Currency (a datatype, which is different from
the Currency format) or Number.
If the datatype is Currency, the maximum number of decimals you can
use is 4. Note: you don't necessarily have to use a Currency datatype
with just money fields. You can then set the field's Format property
to any valid number format.
If it is Number, then look at the Field size property in the lower
panel. It should read Double or Single, NOT Integer or Long Integer
(by definition, an Integer is a whole number and cannot have a decimal
value).
You can then set the field's Format (which effects how the number is
displayed, not how it is stored) and Decimal Places as needed.
Save the changes.