With the recent release of Chrome 56, making the transition from HTTP to HTTPS will wipe out your Facebook Likes because FB sees HTTP and HTTPS as 2 separate sites.

To only way to retain your FB likes is to change the OG:URL content to HTTP

You can fix this by making the following core changes that replace the "HTTPS" in the OG:URL with "HTTP"

Category View:
/public_html/components/com_k2/views/itemlist/view.html.php
line 731 change
from : $document->setMetaData('og:url', $uri->toString());
to: $document->setMetaData('og:url', str_replace( "https", "http",$uri->toString()));

Item View:
/public_html/components/com_k2/views/item/view.html.php
line 569 change
from : $document->setMetaData('og:url', $uri->toString());
to: $document->setMetaData('og:url', str_replace( "https", "http",$uri->toString()));

Latest Items:
/public_html/components/com_k2/views/latest/view.html.php
make the same changes to line 276 (untested)

The only side effect i have seen is the FB Debugger will report a circular redirect path if you also have a 301 redirect in place for HTTP to HTTPS. WHilst FB Debugger reports this as a Warning it does not seem to impact the outcome.

Note: Don't forget to fill in the Meta Information / Description for each Category as this becomes the OG:Description