Invalid Column Name Error in SQL Stored Procedure

I have a stored procedure that I am using to convert tables to a new format for a project.  The project requires new tables, new fields in existing tables, dropping fields in existing tables and dropping an existing table.  The SP takes care of doing all this and copying the data from the tables that are going to be dropped to the correct places.  Everything is working fine except for one table and I can't figure out why.  

For this particular table, it already exists in the database and has new fields added to it.  Then I try and update those fields with values from another table.  This is where I am getting the Invalide column name error.  If I comment out the code where the error is occurring and run the update alone everything works fine so I know the Update statement works.

Here is the specific error message I am getting in SQL Server 2005:
Msg 207, Level 16, State 1, Line 85
Invalid column name 'AssignedAgent'.
Msg 207, Level 16, State 1, Line 85
Invalid column name 'DateTimeAssigned'.

Here is the SP: -- Any help is greatly appreciated!

IF OBJECT_ID('ConvertProofTables','P') IS NOT NULL
      DROP PROCEDURE ConvertProofTables;
GO

CREATE PROCEDURE ConvertProofTables
AS
      SET ANSI_NULLS ON
      SET QUOTED_IDENTIFIER ON
      GO

      BEGIN TRANSACTION
      -- Add new fields to Proof_Agents Table
      ALTER TABLE Proof_Agents ADD
            IsSignedOn BIT,
            MaxAssignedCVLs INT NULL;
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('ALTER TABLE Proof_Agents ERROR', 16,1)
            GOTO _ROLLBACK
      END

      -- Create Proof_Fax Table
      CREATE TABLE Proof_Fax
      (      FaxID INT IDENTITY PRIMARY KEY,
            ProofID INT,
            FaxNumber VARCHAR(10),
            Attn VARCHAR(50),
            CallbackNumber VARCHAR(50),
            UserCode VARCHAR(10) NULL,
            Retries INT NULL,
            FaxStatus VARCHAR(255) NULL,
            FailureReason VARCHAR(255) NULL,
            PagesSent INT NULL,
            TransactionID VARCHAR(19) NULL,
            IsCompleted BIT,
            LastDeliveryDate DATETIME      );
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('CREATE TABLE Proof_Fax ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Add new fields to Proof_Policy
      ALTER TABLE Proof_Policy ADD
            AssignedAgent INT NULL,
            DateTimeAssigned DATETIME NULL,
            IsImaged BIT;
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('ALTER TABLE Proof_Policy ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Create Proof_Submitter Table
      CREATE TABLE Proof_Submitter
      (      SubmitterID INT IDENTITY PRIMARY KEY,
            ProofID INT,
            SubmitterEmail VARCHAR(50),
            DateTimeSubmitted DATETIME      );
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('CREATE TABLE Proof_Submitter ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Pull Fax #, Attn, From Proof_Policy and iscompleted and datetimecompleted from
      -- Proof_Assigned and insert into Proof_Fax
      INSERT INTO Proof_Fax
            (ProofID, FaxNumber, Attn, CallbackNumber, IsCompleted, LastDeliveryDate)
            SELECT PolicyID, Fax1+Fax2+Fax3 AS FaxNumber, Attn, '' AS CallbackNumber, Iscompleted, datetimecompleted
            FROM Proof_Policy
            INNER JOIN Proof_Assigned ON Proof_Policy.PolicyID = Proof_Assigned.proofID;
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('INSERT INTO Proof_Fax ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Pull submitteremail from Proof_Policy and datetimesubmitted from Proof_Assigned
      -- insert into Proof_Submitter
      INSERT INTO Proof_Submitter
            (ProofID, SubmitterEmail, DateTimeSubmitted)
            SELECT PolicyID, SubmitterEmail, datetimesubmitted
            FROM Proof_Policy
            INNER JOIN Proof_Assigned ON Proof_Policy.PolicyID = Proof_Assigned.proofID;
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('INSERT INTO Proof_Submitter ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Set IsImaged to False for all incomplete CVLs
      UPDATE Proof_Policy
      SET IsImaged = 0
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('UPDATE Proof_Policy IsImaged False ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Set IsImaged to True for all completed CVLs
      UPDATE Proof_Policy
      SET IsImaged = 1
      WHERE Proof_Policy.PolicyID IN (SELECT PolicyID
                                                      FROM Proof_Policy, Proof_Fax
                                                      WHERE Proof_Policy.PolicyID = Proof_Fax.ProofID AND Proof_Fax.IsCompleted = 1)
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('UPDATE Proof_Policy IsImaged True ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Set assignment info in Proof_Policy table
      UPDATE Proof_Policy
      SET Proof_Policy.AssignedAgent = Proof_Assigned.assignedto, Proof_Policy.DateTimeAssigned = Proof_Assigned.datetimeassigned
      FROM Proof_Policy, Proof_Assigned
      WHERE Proof_Policy.PolicyID = Proof_Assigned.ProofID
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('UPDATE Proof_Policy ERROR',16,1)
            GOTO _ROLLBACK
      END

      -- Remove Fax info and Submitter Info from Proof_Policy Table
      ALTER TABLE Proof_Policy DROP
            Fax1,
            Fax2,
            Fax3,
            Attn,
            SubmitterEmail;
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('ALTER TABLE Proof_Policy DROP ERROR',16,1);
            GOTO _ROLLBACK;
      END

      -- Drop the Proof_Assigned Table
      DROP TABLE Proof_Assigned
      IF @@ERROR <> 0
      BEGIN
            RAISERROR('DROP TABLE Proof_Assigned ERROR',16,1)
            GOTO _ROLLBACK
      END

      IF @@TRANCOUNT <> 0
      BEGIN
            COMMIT
      END

      _ROLLBACK:
      IF @@TRANCOUNT <> 0
      BEGIN
            ROLLBACK
            GOTO _DONE
      END

      _DONE:
      GO