Question

What's the difference between updating new weeks and updating all weeks?

Answer 1

updating new weeks will only update your new week, while updating all weeks will update all of your existing weeks again, so it will take a lot longer

Answer 2

"new weeks" - for weeks that were not updated yet.
"all weeks" - your whole charts, from beginning to most recent weeks

the "all weeks" option it's useful when you had some struggle with some track, like a tag issue or so... if you had to delete that song from last.fm, you got to update weeks on zero too.

also, it is needed if you want to change the starting day on your charts (Sunday or Friday)