It's less changing the details though than the... Tone. It's hard to explain precisely, but there is a certain tone to the norse myths, grim, yet humorous and very funny. Marvel's version has a... Different tone? Very much more standard superhero/fantasy.

It's not *really* making Sif a warrior, as much as changing the underlying tone/language. (especially, and I'm glad they've tried to distance themselves a bit, making the gods speak shakespearean english. The connotations of shakespearean english and the prose of the Eddas can't really be farther apart)