Invasion, yes.
Genocide, not usually.
Intermarriage and servitude was the more common strategy.
It depends on the numbers. If the invaders/conquerors are small in number, genocide isn't practical because there aren't enough hands to go around to run the farms. If invaders have the numerical advantage, either genocide or whole-scale displacement is the norm. Saxons were a minority with respect to Celts, just as Normans were a minority with respect to both, so subjugation was the strategy that made sense.