Or just a food crop that grows better in Scandinavia than barley and other historical food crops. Maybe it grows even better elsewhere but not enough to displace southern staples, so it'd narrow the gap between Scandinavia and the rest.
Your best bet there is actually the potato. While it doesn't grow any better in Scandinavia than further south, its vulnerable to a lot of diseases, blights etc that doesn't do well in Scandinavian climes.
Thats not enough to make a difference, though. You'd have the somewhat more occasional famine in more southern climes, but it won't be enough to change your basic demographics. Might give an opening for an invasion. At the end of the Roman empire, the Goths, Vandals, etc cared fuck-all about demographics, they just steamrollered the mediterranean. Which shows what you can accomplish when the opposition haven't gottent their act together.
But the centre of gravity, again, changed instantly to the south.
The advantage of early potato introduction is that innovations spread more slowly in pre-indoeuropean times. You're going to have a longer window before the crop is adopted further south than you will later.
Of course, a potato introduction that early almost borders on ASB.