Are there any examples of language shift, where population A changes language because of contacts with population B? I am not looking about examples where population A was conquered or colonized by B. I am looking for examples where population A had an interest in changing to the language of population B, because of extensive contacts, social status etc.
There are many examples of languages spreading more through cultural influence than through the force of arms: Koine Greek is an important cultural example for Western civilization. However, Koine Greek wouldn't have spread so quickly through Asia Minor without the conquests of Alexander. Likewise, the Louisiana purchase arguably reflects at least partly the vast military imbalance between France and the United States (just look at what happened to Texas and California). If you want a clear cut example of a language undergoing a radical evolution purely under cultural influence, you can look at the evolution of prehistoric Japanese to Old Japanese to Early Middle Japanese under the influence of Chinese (mostly through the spread of Buddhism). The grammar, lexicon and phonology of the Japanese language changed quite radically in a century or so in order to incorporate the cultural, literary and religious influence of China. No military component at all was involved.
There were some towns in Northern Australia and Eastern New Guinea where the native Austronesian peoples switched from their native languages to Polinesian so as to better comunicate with the sea-faring traders (who were usually Polinesians).
Swahili also expanded enormously in Eastern Africa once it became perceived as a language of trade, reaching far away from the area where the Swahili people (with their mixed Bantu-Arab heritage) natively resided. Though in Swahili's case its expansion meant it was "added" to the languages the villagers' merchants (usually polyglots) would learn, as opposed to obliterating them.