captivate 8.0 subscription free upgrade to captivate 9.0?

Normally on Subscription you can have two versions of the application: this means it is possible to have both Captivate 8 and  9 on your system (contrary to what pbdw... claims). That is the same for all subscription plans (also CC). You should do so, because once you upgrade a project from 8 to 9, you'll not be able to open it again in version 8. Since there can always be upgrade issues, you should keep the CP8 version safe.

So I strongly recommend not to 'update' but to install the trial version of CP9, along CP8. When you log in with your Adobe ID, which is linked to your subscription license, you'll be able to use CP9.

As for El Capitan: no version of CP is compatible with it, CP8 and CP9.

Also be aware that Cp 9 is NOT compatible with the current version of the Mac OS (El Capitan).

I believe if you are eligible the update will appear in the Help > Updates... drop down menu. I have a perpetual license presently but my understanding was that one of the beneifts of the subscription model is that you could always update to the latest version. Keep in mind that you may not roll back to 8.0 once you do this. You may wish to wait until you are finished all your current projects.