Do you mean photoshop cs (photoshop 8) or photoshop elements 8?
Which operating system are you using?
Photoshop CS. Was running on system 8.6 but just upgraded to Yosemity.
Yosemite does not support PPC Universal applications. Your Photoshop CS 8.x does you no good, with or without a CD.
Unfortunately, photoshop cs will not run on mac os x 10.10 Yosemite, because photoshop cs is a PowerPC (type of processor) application and Yosemite uses an intel processor that can't run PowerPC applications.
(apple removed Rosetta after os x 10.6.8 snow leopard)
You can buy photoshop cs6 or subscribe to the creative cloud for photoshop cc 2014, if you want a version of photoshop that runs on your new operating system.
(adobe has the Photography package which comes with both lightroom and photoshop for around $10 a month)
Is it easy to reinstall my 8.6 over 10?
No, Mac OS 8.6 was designed to run on a Power PC Mac.
SInce you are now running OSX 10.10 Yosemite you obviously have an Intel Mac.
You have a total incompatibility between your hardware and your software (both Photoshop CS & Mac OS 8.6)