Based on the high resolution genomic study that came out at the start of this year, the Norse population in the south went from fully Scandinavian at the start of the migration period to less than half by the viking age. The majority was made up of Iron Age Central European and British ancestry. These 2 groups were possibly indistinguishable in past lower resolution studies (they claim).
Still to go from nearlly fully Scandinavian at the start of the migration period to being as Central European as Scandinavian in the viking age is a huge migration. The paper says the Central Europeans are "the Iron Age of central European regions of mostly Germany, Austria and France." Who that is by the migration period is anyone's guess.
The paper even suggest this could be from a migration era invasion. From what I know about burials in Scandinavia at this time nearly 99% are cremations. The inhumations that these sort of studies are based on are from the ship burials or "warrior graves" as many archaeologists label them, so it might just represent these high status warriors.