Average rolls for 2d10 and 1d20 are very different. 11 vs 10.5. So if you're going to double it, it would work better to be consistent and make it 2d4, 2d6, 2d8, 2d10, 2d12. On the other hand, lots of things are dependent on Hit Dice, so doubling their HD would break things. And I doubt many people have d16s lying around, so doubling the number doesn't work too well either.
I think that making their HD become d12s and giving them Unholy Toughness is fair.