Yes, if you don't plan to use your previous Reason version after upgrading to Reason 12, it's perfectly safe to uninstall it.
Proceed as follows:
- Select your previous Reason version in the "Start > Settings > Apps > Apps & features" list and then click the Uninstall button.
- In Windows File Explorer, go to:
%Programdata%\Propellerhead Software\Soundbanks
and delete the "Reason x" folder, which contain the sound banks for the older version. 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!