Stored procedure parses correctly but will not execute. Invalid object name. Msg 208 - sql

Question

I've scripted up a stored procedure as follows. It will parse without errors, but when I try to execute it, it will fail. The error message reads: Msg 208, Level 16, State 6, Procedure aspnet_updateUser, Line 23
Invalid object name 'dbo.aspnet_updateUser'.
Here is the stored procedure.
USE [PMRS2]
GO
/****** Object: StoredProcedure [dbo].[aspnet_updateUser] Script Date: 05/25/2009 15:29:47 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author: <Author,,Name>
-- Create date: <Create Date,,>
-- Description: <Description,,>
-- =============================================
ALTER PROCEDURE [dbo].[aspnet_updateUser]
-- Add the parameters for the stored procedure here
#UserName nvarchar(50),
#Email nvarchar(50),
#FName nvarchar(50),
#LName nvarchar(50),
#ActiveFlag bit,
#GroupId int
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
-- Insert statements for procedure here
UPDATE dbo.aspnet_Users
SET UserName = #UserName, LoweredUserName = LOWER(#UserName), Email = #Email, FName = #FName, LName = #LName, ActiveFlag = #ActiveFlag, GroupId = #GroupId
WHERE LoweredUserName = LOWER(#UserName)
END

Answer 1

Looks like it might not exist yet, swap the Alter to a Create.

Answer 2

To avoid this happening in the furture, do what we do, never use alter proc. Instead we check for the existance of the proc and drop it if it exists, then create it with the new code:
IF EXISTS (SELECT * FROM sysobjects WHERE type = 'P' AND name = 'myProc')
BEGIN
DROP Procedure myProc
END
GO
CREATE PROCEDURE myProc
(add the rest of the proc here)

Answer 3

Here is another solution
USE [PMRS2]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
IF OBJECT_ID ( 'dbo.YourProcedureName', 'P' ) IS NOT NULL
DROP PROCEDURE dbo.YourProcedureName;
GO
CREATE PROCEDURE [dbo].[YourProcedureName] (
#UserName varchar(50),
#Password varchar(50))
AS
BEGIN
SET NOCOUNT ON;
select ... (your query)
END