That's right, so I replaced Declare with Declare Ptfsafe as below in 2 places in the frmMain code and it works again:
Private Declare PtrSafe Function FindWindow Lib "user32" aka "FindWindowA" _ (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare PtrSafe Function SetWindowPos Lib "user32" (ByVal hwnd As Long, ByVal hWndInsertAfter As Long, ByVal x As Long, ByVal y As Long, ByVal cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long
I knew that there were compatibility problems due to the versions but I hadn't thought about that.