Is there any way of converting an existing field to a Matrix? I'm working on an existing site that has an "Entries" field that allows authors to provide a selection of related articles. However, since some articles have titles whose meaning is not entirely clear in this different context, the client would like to be able to tweak the wording of the titles in this "related articles" context.

The best way I can see to do this would be to make the "Related articles" field a Matrix, with each block containing both an entry and an alternative title. However, this is an existing site with lots of posts that already use this field, so I'd need to somehow convert the existing "Entries" field to a Matrix. Is this possible at all?

(I realise that an alternative approach would be to add a "Short title" field to entries. However, UX-wise this feels awkward compared to the Matrix approach, since it would require the editor to also potentially edit each of the related articles, which is both more long-winded and less clear as a process)
Unless each time an Entry is selected the name needs to be different (to other times that same Entry is chosen), then I would suggest using your latter approach of adding an secondary Title field as it would be more efficient in the long term because the secondary Title would only need to be added once. 

If it's about an Entry, consider keeping it 'with' the Entry. 

I use a similar approach for Meta Titles to override the Entry title for SEO purposes.