Substitution in vim is, by default, case-sensitive. Adding the i s-flag
makes it case-insensitive.
vim-abolish, on the other hand, lets
you perform a case-insensitive substitution that preserves three case
variants (foo, Foo, and FOO). Substitution with vim-abolish can be
performed with Subvert or S.
For instance, :%S/blog/article/g will turn
blog Blog bLOg BLOG
into
article Article bLOg ARTICLE
Install vim-abolish and see :h Subvert for more details.