However, if an open access version of a text is available, this must always be treated as the primary text. Here the commercial version of the text becomes the secondary version and it should always be cited second and in a manner that makes this completely clear. For instance, after the primary reference to the full text, you could write: ‘Also available as: ….’
Would be interesting to write a tool that could take a paper as input and replace all citations with references to freely available versions