Enamul Hello,
You trip to Milan will be possible. If all Glacier Express trains are sold out, you can also take regular trains on the same line, for these trains no seat reservations are mandatory.
The problem is that your tickets could be valid only on the 9:52 AM train and the connecting train to Italy. I made a test with the same date and time, it seems that tickets for Switzerland are flexible and valid for every train, but I give no guarantee. I suggest to call the SBB Contact Center to clarify this point:
011 41 51 225 78 56
(available 24/7, sometimes you may be required to wait)