It might mean your doctor has just got an expense-paid trip to Hawaii to listen to an CME lecture on Metformin from the manufacturer. Or perhaps the old drug rep retired and the new one is a slim tall blond who wears tight blue jeans. And sometimes it has to do with insurance reimbursement. Metformin and glyburide are both relatively safe drugs with good historys used in moderation in with your physician's instructions, but each acts in a different way. Some physicians administer both at the same time. Another school of thought is to use one for a while, and then switch. I would ask him, but the change does not necessarily mean something bad has happened. You can look up the drugs. I have no strong feelings one way or the other.
There are a lot of reasons why a doctor might want someone to change medication; did you ask your doctor why? How are your blood sugars? If they are stable, then your diabetes is "not getting worse". I'm assuming you test your blood sugar regularly.
But just FYI diabetes IS a progressive disease, and sometimes a medication stops working (keeping our numbers in target range) or it is necessary to add another medication. It doesn't mean we are doing anything wrong.