You can put something like this in a stored procedure and provide some functionality to call it in the front end:

----------------------------
CREATE PROCEDURE ChangeTaxRate @newrate decimal AS

IF OBJECT_ID('dbo.CurrentTaxRate') IS NOT NULL
EXEC sp_unbindefault 'CurrentTaxRate'
DROP Default dbo.CurrentTaxRate

CREATE DEFAULT dbo.CurrentTaxRate AS @newrate
EXEC sp_bindefault 'dbo.CurrentTaxRate', 'Customer.TaxRate'
----------------------------

I just kind of made it up on the fly so if you find this approach useful you will have to fiddle with it a little.