What made these nordic conquerers give up their gods in exchange for this new religion? What was/were the triggers?
Scandinavians tell me it was via force - you either converted or were killed. Meanwhile, my Danish lit teacher posits that some German missionaries somehow managed to convince some Vikings to convert, and Catholicism slowly spread out and became more popular until it eventually replaced the Vikings' religion.
Edit: by Vikings, I mean from Scandinavia
It is true that both aspects respectively by your Scandinavian friends and by your professor were both present in the process of 'Christianization' (I avoid to use the term 'conversion' here, as explained below) of the Scandinavians during the Viking Age, and it also heavily depends on your definition of 'the Vikings', either in their diaspora like Normandy in Northern France or North-Eastern England where they almost inevitably got in contact with their new indigenous Christian neighbors or in their homeland. I rather incline to emphasize the latter perspective, i.e. long and gradual infiltration of Christianity before the final and crucial transition phase where some violence as well as coercion could sometimes be involved as your friends told you mainly based on the later narrative sources, but not always so.
The point to understand the complex relationship between pre-Christian Norse beliefs and early medieval Christianity here is that both 'religions' were probably not binary for many contemporary Vikings, and both concepts of respective 'religion' did not correspond fully with the modern definition of religion familiar to most of us. I avoided to use the term 'conversion' above since this word is primarily based on the individual religious identity of a person, but it is really difficult to deduce such a concept from early medieval textual sources, regardless either of pre-Christian as well as of Christian ones. Resistances to one religion, represented in sources, were rarely based on purely 'religious' motive. It often involved with some political and social conflicts, such as the opposition against the repulsive new much rigorous 'Christian' ruler.
Even the 'right' set of beliefs in each medieval 'religion' is not so easy for us to define or to understand: What we have in our hand as sources of pre-Christian Norse religion should not be understood as a pagan canon counterpart of the Bible. They were recorded in the 13th century Iceland, more than two centuries after their 'official conversion' to Christianity and inevitably include Christian or Classic inspired interpretation of their past. As for more details on this issue, I'd recommend /u/Platypuskeeper's excellent answer in How much do we actually know about the ancient religious traditions, deities and narratives of the Norse?. Even if the author could convey use the relatively precise information on some points, it would also be, so to speak, a tip of the big iceberg of pre-Christian Norse beliefs, with considerable regional or social differences, as testified by the diversity of archaeological finds like burial practices. These beliefs also concerned rather with the communal rituals like seasonal sacrifice feasts under the guidance of local chieftains than with the individual faiths. Some Scandinavian researchers even compare them rather as 'customary rules [of the community] (Old Norse sið)' than as a religion.
Surprisingly enough, the majority of the Scandinavians (especially rulers and early town dwellers) in the 9th century seemed to rather willing to tolerate Christians in their society as well as Christianity itself as long as they did not pose the serious threat against the social order. They were even not so hesitant to incorporate a new Jesus as someone new into their ill-defined pantheon (Meulengracht Sørensen 1997: 223). Some of them had indeed little difficulty in being a Christian and a 'Norse pagan' at the same time. Carolingian biography author, Norker the stammer, wrote in his work on Emperor Louis the Pious of the Franks (d. 840), an episode of an old Dane who had come to visit a royal palace in Frankish Empire more than twenty times spontaneously to get baptized in exchange of fine clothes (Notker Baldus, gesta Karoli Magni imperatoris, II-19: Cf. Noble trans. 2009: 115f.). I don't say his understanding of Christianity was orthodox in the 9th century Europe (certainly the author ridiculed him as well as the Danes in general), but he at least didn't hesitate at all to accept the baptism that offered him exotic prestige in splendid court rituals as well as a kind of '(one-time?) passport' in early medieval Christian kingdoms.
Some Frankish and Anglo-Saxon rulers requested the Vikings to agree the baptism in exchange of their concession like the payment of tribute and the approval of settlement. This was actually win-win relationship for both parties involved: The ruler could showed off their generosity and at least formal 'religious supremacy' to the Vikings in religious fictive kinship (god parent - god child) as well, and the Vikings in turn could get real wealth and prestige as successful band leader as well. It would have been also much beneficial to being (at least one-time Christian) to negotiate with their Christian neighbors as well as to trade with them (So I dared to compare the baptism as a passport above). As a result of this kind of long and repeated interactions between Europeans and Norse people during the Viking Age, more and more Norse people increasingly saw Christianity with favorable way, associated often with exotic wealth and prestige, and it is likely that some of them kept Christian practices also in their Scandinavian homeland. As I mentioned briefly in We often see the results of Viking/Scandinavian invasions/colonization on other lands like those of the modern British Isles, France, and Italy, but what effects did they have on the Scandinavian homeland?, some new ambitious ex-Viking leaders those who accumulated their wealth in Europe must also have found this new exotic religion rather useful: They tried to take the place of traditional local chieftains who had presided seasonal communal rituals for long, so Christianity and their clergy was for them a kind of auxiliary weapon to undermine their rival's political as well as religious authority.
Then, the final moment would come around the end of the first millennium: The transition of representative rules of the political community from pre-Christian Norse beliefs to Christian ones, i.e from the Solstice seasonal feasts like Jule to Christian practices like Christmas. The more rigorous Christian-based social order would be established, and such a new society, based on the monotheism, was much less tolerant against religious minorities than their predecessor, Old Norse belief based one. Now those who had so far enjoyed some dual 'religious identities' of being a pagan and a Christian, such as an old Dane mentioned above, faced a hard decision of abandoning their older beliefs for ever, i.e. gave up the communal feasts to band them together. Not all the new 'Christians' or such pagan-Christians were so willing to do so with much certainty, so some kind of conflict were recorded at that critical phase.
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