We just went over 10,000 in 2 funds in our Vanguard Roth after our 2015 contributions, and a week or so later I received a notice saying that the shares were eligible for upgrade if we wanted. But we had to actively change it; it didn't happen automatically. I had to fill out some online forms for each upgrade and they gave me the option not to upgrade. It showed up in Mint as transactions: a 'sell' of investor shares and 'buy' of admiral shares.
I'm glad they notified me of the option because I would have forgotten otherwise. If we hadn't done it, would it happen automatically at some point or is it always elective? And would there ever be a reason to decline?