The distribution of names you sometimes see - Celt for natural features, Saxon (and Viking) for manors & trading places show us populations weren't suddenly displaced.
It's not as simple as that though. Temporary alliances with Saxons, motives for the attack, multiple targets...
Don't know whether they'd consider it invasion, or reprisal.
You can see it looking at the map. I used to live in Great Yarmouth, in the middle of a cluster of Viking place names.
For every warband that burned down a burh & fled with some gold, other settlers migrated, settled and became part of our story.