[inglor_cz]

There is a continuum between transmission and replacement.

The Japanese adopting democracy after WWII was almost pure transmission; the population stayed basically the same.

The Holocaust was almost pure replacement; only a few people from mixed families survived in some areas.

But "something in between" is much more typical. Contemporary Iran is as much a heir to ancient Persia as it is to the Arab conquest and Islamization. Roman conquest of Britain led to a significant population admixture from the rest of the Empire and Romanization of local Celts at the same time. Expansion of Germans through Central and Eastern Europe in 12th-18th century was a thorough mix of replacement and transmission.