Updating old Captivate to Captive 8?

The first thing you need to be aware of is that if the old Captivate files are from a version TOO far in the past then you will not be able to open them at all.  Cp8 should still be able to open Cp6 files, and you may be lucky if the files are Cp5 or 5.5 but you can forget about trying to open any of the older Captivate project files that are .cp rather than CPTX format. (I think CP6 may have been the last version that could open Cp4 files.)

Once you CAN open and upgrade a project file, the next thing you're going to find is that there will be quite a lot of re-work involved fixing things that have changed over the years.

If you are planning these upgrades because you wanted to move the courses over to HTML5 then it gets even more complicated.  Take a look at this article:

Converting Adobe Captivate SWF courses to HTML5 | Infosemantics Pty Ltd