Change 2-space indent to 4-space in vim

A general way of changing the indent is by changing the tabstop:

Paste your file into an empty buffer, then:

:set ts=2 sts=2 noet
:retab!

This changes every 2 spaces to a TAB character, then:

:set ts=4 sts=4 et
:retab

This changes every TAB to 4 spaces.

The advantage of this method is that you can also use it the other way around, to convert from 4 to 2 spaces for example.