Private Sub Project_Open(ByVal pj As Project)
  EnableEvents
End Sub

Option Explicit

Public Chng As clsChange
Public Ev As clsEvents

Sub EnableEvents()
   Set Chng = New clsChange
   Set Ev = New clsEvents
End Sub

Option Explicit

Public Event AfterChangeTask(TskID As String, _
                       ByRef blnCancel As Boolean)
Public WithEvents ProjApp As MSProject.Application

Private Sub Class_Initialize()
   Set ProjApp = Application
End Sub

Private Sub ProjApp_ProjectBeforeTaskChange(ByVal tsk As Task, ByVal Field As PjField, ByVal NewVal As Variant, Cancel As Boolean)
   If NewVal = "Значение1" Then
      If MsgBox("Are you sure you want to Change Task" & _
         vbCrLf & vbCrLf & tsk.Name, vbYesNo) _
            = vbNo Then
        Cancel = True 'Cancel the Task Delete
      Else
            tsk.PercentComplete = 100
            RaiseEvent AfterChangeTask(tsk.ID, False)
        ' End If
      End If
   End If
End Sub

Option Explicit
Public WithEvents EventApp As clsChange

Private Sub Class_Initialize()
  Set EventApp = Chng
End Sub

Private Sub EventApp_AfterChangeTask(TskID As String, blnCancel As Boolean)
MsgBox ("Ок!!")
End Sub