Can I delete my previous Reason version after upgrading to Reason 12?

Yes, if you don't plan to use your previous Reason version after upgrading to Reason 12, it's perfectly safe to delete it.

Proceed as follows:

Delete the old "Reason x" folder in Applications.
Then go to: /Library/Application Support/Propellerhead Software/Soundbanks and delete the old "Reason x" folder, which contain the sound banks for the older versions. These are not needed for Reason 12.
The only folders that are required for Reason 12 are the "Reason 12", "ID8" and "Orkester" folders.
Done!