Older .cp files needing converted to .cptx. Need trial of Captivate 5.5 or similar to do so

CP-files are very old (version 4 or older). I really doubt that it is worthwhile to try to upgrade to a more recent version to be able to publish to HTML5. Will try to explain the reasons. Those files may still have elements based on ActionScript 2 which was no longer supported since version 5. Output to HTML5 appeared with CP6.1 and has improved with each later version, with lot of added features. I cannot exactly remember if the cp-files had a Library, but if they had, it will not be possible to open the Library as external Library in more recent versions at all.  For sure they were not based on a theme, which started only with Captivate version 6. You could try to unzip the cp-files and extract the assets which are still usable. But I would recommend to start from scratch in the recent version you have now (which one?). It may seem more work but probably will prove to save time compared with having to upgrade through different older versions (maybe version 6, 7, 9) to see that almost nothing is working at all in HTML5. Images and text are treated differently, rollovers cannot be used, nor can Text Animations, Slide transitions, animations in SWF format, videos in FLV format. You may even end up with used fonts which are not available on your system and cannot be replaced with one click as is the case in version 11.5. Building a theme in the recent version, choose a resolution that is more appropriate for the presently used devices, or maybe choose immediately for a responsive project development? Depending on the type of project, you could play the project when published (you need the original version for that indeed) and record with Video Demo and use those video clips directly in a recent project, or even turn it into an interactive video.

My apologies if this is not really the expected answer.