what's the best/optimal/safest/recommended way to proceed?
also, if clean install is way, should we first UNimmunize in 1.5.2 before uninstalling it?
I recommend that you do a clean install:
First disable any protection within Spybot S&D (i.e. Teatimer, SD Helper) and undo the immunisation. You can also delete any back-ups in the Recovery tab (but this is optional, and I don't recommend it!). Uninstall Spybot S&D and then you can proceed and install Spybot S&D 1.6
Spybot-S&D will uninstall from the Windows Add/Remove Software control panel without problems.
If you want to completely get rid of Spybot-S&D and the Add/Remove does not help, you can delete the installation folder (usually C:\Program Files\Spybot - Search & Destroy\), then download this very small fix and execute it (important! - this will remove all remains from the registry).
If you just want to upgrade to a newer version, please follow the same instructions like above and then install the new version.
After following these instructions please restart your system so that the changes can take place.
Also, neither the automated uninstall nor the manual uninstall like described above will remove the following directories, which you will have to remove by hand:
Windows 95 or 98: C:\Windows\Aplication Data\Spybot - Search & Destroy\
Windows ME: C:\Windows\All Users\Application Data\Spybot - Search & Destroy\
Windows NT, 2000 or XP: C:\Documents and Settings\All Users\Application Data\Spybot - Search & Destroy\
Windows Vista: C:\ProgramData\Spybot - Search & Destroy\