converting captivate course from flash to html

You seem to be lucky enough to be able to open those legacy courses in CP2019 because that is not always the case.  As preparation for a webinar, which finally was given by someone else (because my topic was changed) I created a survey. That survey leads to three possible scenarios. Have a look at:

http://blog.lilybiri.com/using-cpquizinfoanswerchoice-for-survey-conversion-from-swf-to-html

If you have the same luck with the old files, the HTML tracker can help you with non-supported features. But be careful, it is not perfect. Example: rollover caption/image are supported on devices with hover events (desktop/laptop) but are listed as incompatible with HTML5.

If you cannot open the files in the present version, know that in most cases it is still possible to open its Library as external library if you need to recreate the file. I have opened libraries from courses created with Captivate 5.5.

If you don't have the cptx, but only the output files, a recording with Video Demo and adding interactivities to create an interactive video can be a solution if the number of interactivities is limited. It is always possible to add extra slides with scored Questions and/orDrag&Drop.

Based on my experiences it is not at all 'just republishing'... much more needs to be done.

That depends - do you have the original development files (.cptx) or just the finished course packages? If you have the original dev files, it's simply republishing them (and probably re-testing them, of course).

If you don't have the original files, I don't think you can update them without rebuilding 😞