Yes, a sell to buy transaction is a taxable event. You should NEVER sell investor shares to buy admiral shares. Instead, Vanguard will CONVERT your investor shares to admiral shares, which is NOT a taxable event. If in doubt, call Vanguard and have them do it for you. Never, ever just hit the sell button. I've heard of people doing this and Vanguard could probably do a better job of making the difference more clear.
Eventually, Vanguard will convert them automatically but it takes a while.