How much did the monks who wrote "Beowulf," change the story to fit Christian views?
Think about it this way: The monks were the scribes...the printing press of the time period. Since they were religious, they would change anything they considered not worthy of attention by readers--current or future. This means all dirty jokes, anything vulgar or obscene, and anything they considered too common. So, since they were the recorders of the oral literature of the time (since so many were unable to read and write for themselves, the songs, poems, stories, riddles, jokes, etc. were recorded by the monks), they chose what to write down and keep for posterity and what they would ignore. Much was lost because of this, and it stands to reason that of the written literature we do have, much was changed.
Monks were missionaries of their time. Because the Monks were scribes they changed a few things in "Beowulf" in order to allow them to relate to the Anglo-Saxon people. The Anglo-Saxon people believed in Paganism and therefor without the changes in "Beowulf," relating to them would be difficult
As much as any scribe or translator of history or literature oral or written, bible, vedas, ancient and current as well; modern cinema is constantly remaking movies of a bygone era often adding new elements. There is no doubt that audiences in ancient times grew tired of the same old same old just as we do today. If you read Tom Sawyer a hundred times and then some one asked you to translate it into a new language you would likely find it hard not to reflect subtle changes as most languages have different actual connotative meaning for words that may seem the same in function. Omitting these types of details is often the worst case, rather than leaving some trace, confusing as they may be, to the origin for future analysis. Our ancestral editors have shaped our views of the past to be sure, how much so, is almost impossible to quantify.