I normally use spaces in my files instead of tabs for indentation, with 4 being my default indent width. Sometimes, I need to change this on existing files to 2, so for the most part, the following sequence works. First,
set ts=4 sts=4 noet retab!
This changes every 4-space to a tab, then retabs (exclamation point is important).
Then, change the tabs into spaces:
set ts=2 sts=2 et retab!
You can also setup your vimrc to have certain files like HTML use a different tabstop than your regular files:
autocmd FileType html :setlocal sw=2 ts=2 sts=2