Yeah, sure:

Select the profile you want, click the arrow at the bottom left, which makes a menu appear. In that menu, select Move/Personalize and you can change the data you like

Oh sorry, my bad.

Purchase date can't be erased, indeed; but a work-around might be moving that profile back to the wish list, in which case DVDP automatically removes the purchase information, including the date.

However, if you wish to keep it in your owned list without a purchase date, then I think for now that's not possible, sorry.