I made the switch last year from PC to Mac, I was running CS4 on my PC. Just downloading the Mac version and using your existing serial number won’t work as far as I know. What I did was ring Adobe and explained what I wanted to do, they sent me a form I had to fill out and sign saying I would destroy my PC version of CS4. The mac Mac version was posted out to me after that, there was a €10 shipping fee if I remember correctly. Certainly beats having to buy the whole thing again though. Hope this helps.