0) I've converted an ext3 filesystem to ext4 by (in short) running tune2fs followed by e2fsck.
1) The tune2fs command I used was:
tune2fs -O extent,flex_bg,huge_file,uninit_bg,dir_nlink,extra_isize $DEVICE
These features were simply all features enabled for the ext4 filesystems on the same disk which weren't enabled for the ext3 filesystem I wanted to convert. (These ext4 filesystems were created by Fedora installer tools. The last installer I used was for F16. I can't remember tuning these ext4 filesystems manually.)
2) Of these features a number are not mentioned in man 8 tune2fs:
3) To me it seems these three features should either:
be documented in man 8 tune2fs; or
not be allowed to be set via tune2fs (in case setting them via tune2fs is not useful or even dangerous).