If you run them through a 30/06 FL sizer die they will. Yes they will need to be trimmed.
The shoulders start at the same postion on both cases, because of the smaller diameter of the 270, the shoulder angle is longer, when you expand the neck to .308, it will shorten the angled part of the case, the angle itself will be the same.
Use some inside neck lube when you expand.
Jimmy K