Got a site that due to bad design never had SEFs enabled. Wondering if I enable them, would all the site's current backlinks linking to non-SEF urls still correctly return the pages or would they return 404?
Direct links to a Joomla article will always work, and Joomla is using it itself to link between articles.
For example, this URL:
http://www.website.com/index.php?option=com_content&view=article&id=53:abu-sidra-14-1-looks-value-each-way-bet-at-meydan&catid=17:previews&Itemid=102
is the native one and is giving instructions to your Joomla website to access the article database, so it will always work no matter what, even though the SEF URL is:
http://www.website.com/betting-previews/53-abu-sidra-14-1-looks-value-each-way-bet-at-meydan
However, any alternative SEF URLs which may have been done on the website will not work, e.g. the above SEF URL example with category number (53) removed by some SEF tool.