Opening Old Layered PDFs in new illustrator version

Hi,

For the last 8 years I've been creating documents where I save layered PDFs as the final version.  Often I am required to revisit these layered PDFs to amend items on individual layers.  

I have noticed that when people create these documents in the latest version of illustrator and save it as a PDF the layers are no longer visible in my slightly older illustrator programme. It apears as a flattened file.  I'm happy to update illustrator and use Ai. files but how can I retain access to the 8 years worth of old documents with layers.  I can't go through and resave all of them as Ai. files (there are 1000s). Is there a way I can switch to an earlier version of illustrator (whilst still having a new version installed) so that I can continue to amend the PDFs?  

As mentioned, I'm happy to save Ai files going forward, but I need to be able to edit layers on old PDFs. 

Thanks for any insight.