How to modify ENUM datatypes?

All,

In the past I have used foreign keys to lookup tables for small lists of
values that I now think ENUM could do the job of.  I was hoping that by
using ENUM, I would avoid having to do joins in my queries, and that I'd
be making the data more robust and faster.

I used to have a table for account_status:

   A | Active
   B | Billing Failed
   C | Closed
   D | Deactivated

   account.acct_type CHAR references account_type.acct_type CHAR

But, now I've converted that to an ENUM:

   ACTIVE
   BILLING_FAILED
   CLOSED
   DEACTIVATED

   account.acct_type ENUM account_type

The problem is that once I create a column in my account table that uses
this 'account_type' datatype, I can't seem to change or add to it any
more.  I want to add a new value or edit/delete an existing one.

How do you make changes to an ENUM datatype that is already in use?

-- Dante