How do you calculate the cost basis when you switch from one share class / NAV of a mutual fund to another? I switched from Vanguard Mid Cap Index to the Admiral Shares of the same fund. I received two dividends under the old fund and then subsequently sold my entire position in the new fund.
A "switch" of funds is usually the same as sale of one stock and purchase of another. (Might be different if it's the exact same portfolio but only your expense fee rate changes.)
Your basis -- including the dividends if you reinvested them in additional shares -- simply transfers over to the new shares.