do you have to turn the necks downas well when going from 308 to 7-08
Have to? I don't know, I've never done that particular case forming, but my guess would be it depends on what .308 brass you're starting with, and how thick its neck is. (and if you are moving the case shoulder) Which is why you should measure, to be sure. Neck turning MAY be needed, I can't say. Measuring the formed case neck thickness will tell you.
And, speaking of moving the shoulder, take a close look at the dimensions for the 7-08 and the .308 cases.
Overall length, the 7mm-08 case is slightly longer (0.02"), the case shoulder is slightly longer (0.04"), but the case neck is slightly shorter than the 308. Both cases are the same from the base to the bottom of the shoulder, but then the dimensions differ.
Like I said, I've never done it, but going by the case drawings, what I'm thinking is, that running a .308 case into a 7mm-08 sizer will squeeze down the nect, BUT, since the 7-08 case neck is shorter and the shoulder longer than the .308, I THINK it will leave a .308 size "ring" at the base of the new 7mm case neck.
IF this is what happens, will it chamber in your 7mm-08? I don't know. If it chambers, I would expect it to fireform and be ok.
I have formed a number of cases from other cases, but not that particular combination, so I can't say for sure. You'll need to find someone who has done it (or do it yourself) to be sure of all the little quirks that might apply.