Can I be charge with both 2nd degree and manslaughter? Does the case have to start with one or the other, can it be changed later?

Unless you killed two people, you can not be charged with both at the same time.

But they can change the charge at any time they want up till the trial starts, ( after that they can only lower the charge) ** normally

But you can be charged with manslaughter now, and as evidence comes in, or as some DA wants to make a name for thierself, they can up the charges. And as done often, they can make a plea bargain to a lower charge

You would typically be charged with one or the other. However, the judge may permit the jury to consider convicting you on a lesser charge. So, for example, if you were charged with 2nd degree murder, the judge may permit the jury to consider the lesser charge of manslaughter as an alternative conviction.