Using 0.13.0rc1 (or rc2?) I emptied my old non-HD wallet, shutdown bitcoin-core, moved the old wallet.dat away and started bitcoin-core again, to get a new HD wallet created. So the new wallet should be a HD wallet, because I did not change the default “-usehd” option value of 1.
But I would like to see a confirmation of my guess somewhere in the GUI, maybe in the debug window, or maybe under Help->About wallet. Maybe also the wallet should have a version number to be able to clearly identifiy the type of wallet (e.g. wallet type version 1 = HD Wallet)