My advice would be to post this in the correct A3 section of this forum, I tried asking a question about upgrading to DAB on an B8 S4 here and did not get much in the way of answers.
By the way, you need to work out which system is in your car and if it is a "low spec" system, like mine was/is, then maybe the easy way is to buy a used DAB version of your current setup, that is what I did, it will probably need Component Protection removed by a proper VW Group Indy or an Audi dealership which is okay as long as it was not stolen. Then you will need to source, again maybe used DAB aerial amplifier and a new Fakra/SMB Female to Female lead. I've completed my "upgrade" and everything works as expected.