So last night I think I fell asleep before finishing the point I wanted to make: these population events lie on a continuum, with total replacement on one end and complete assimilation on the other.
The degree of leaning towards one side or the other can be thought of as more or less probable depending on a number of factors, namely:

Number of Population Difference.
We can easily imagine one (numerous) group having a much larger influence than the other as they come into local proximity due to their population advantage. This can give them power towards replacement, if that path is taken. On the extreme ends we have an individual moving into a population (assimilation or replacement both possible), and a group moving into a region with a sole inhabitant. (either total replacement or assimilation, depending)

larger population groups have more power than smaller ones, a power differential which can lead towards replacement http://www.elrst.com/wp-content/uploads/2008/05/refugees.jpg

Genetic Difference
With two contending groups the degree of genetic similarity will also affect the outcome. Differing species will have a very difficult time absorbing into each other as they are incapable of reproducing together. on the other end, two groups with nearly identical phenotypes (not necessarily genotypes but one does contribute to the other) may not have any conflict and readily absorb.

Very well known graph of neanderthal genetic differences - were they close enough genetically to assimilate? http://www.thefastertimes.com/bones/files/2009/07/genetic-differences-graph.jpg

Cultural Difference
Groups with similar cultures will tend towards assimilation whereas conflict can arise from differing cultures. These differences are readily seen in the archaeological record as differences in burial practices, ornamentation, or toolmaking techniques, though a sharper change is required to differentiate from simple evolution of technology/culture.

Misinterpreted signals can lead to conflict as different cultures interact http://www.1000ventures.com/design_elements/selfmade/ccd_example_ok-gesture.gif

 

One last note on this focus towards area-displacement; Archaeological methods can give us a continuous record in one location but time resolution across sites lies in the order of tens of years at best, so even though it can be argued that these population units may travel from area to area, and it's possible one group displaced another geographically without affecting them otherwise, we can define the events as a changing of access to locational resources, which helpfully constricts our analysis to singular locations.