Cannot Change Referenced UDF

I have a UDF in my database like so...

ALTER FUNCTION [dbo].[f_spn_GetCustomerNo]
(
-- Add the parameters for the function here
)
RETURNS int
AS
BEGIN
RETURN 12441
END

This function is used as the default for a Customer_No column in every table. Now I want to change the UDF to RETURN 11564 but when I attempt to Alter Function I get the error:

Cannot ALTER 'dbo.f_spn_GetCustomerNo' because it is being referenced by object 'DF__tm_spn_eq__Custo__5C6CB6D7'.

which is a table that is using the UDF as a default. Is there any way around this with the exception of removing the default from every table, changing my function, and then resetting the default?

TIA!