How do I deactivate and older version of Photoshop?

You do need to deactivate a current install to open the use of that license for another computer.  There should be 3 installs with your purchase.  If you've installed it 3 times without deactivating one of the installs, you will need to contact Adobe to give you the ability to reinstall the software by giving you 1 more activation.    I've gone thru the process myself.   

The other consideration is that the older CS programs don't always work with a new/update OS, if you get past the original install - you'll probably have issues updating the software.   In that case, you have to manually update the software from the Adobe site, and hopefully the update will work with your OS...  (I'm going thru this issue right now, contacted Adobe this a.m., and they couldn't help me...).  Just told me to read the forums.  You have to update each program individually, and not use Adobe's Update Manager - because that will probably not update your software correctly, if at all...