I am sorry it might be a dumb question, can I upgrade old Nik Collection or just buy brand new one from DXO?
I’m not sure I understand the question. Are you asking if you can purchase the Nik Collection at the upgrade price because you own the earlier Google or Nik version? The answer to that is no. You have to purchase it at the regular price.
Whether you upgrade or pay full price the software you download is identical, and when installed will overlay your existing version. If you intend to trial it make sure you have the original install files of your current version just in case you decide not to purchase the Nik Collection 3. You will need them to reinstall you current version.