I'm just guessing here, and I hope that Joyce or someone from Stellar could correct me if I'm wrong. It could be as simple as announcing that you're going to make this change to de-decentralize, describing what that process entails, and asking all the invested parties (in this case validating nodes) to follow suit. I imagine that the process is similar to when bitcoin has done a hard fork in the past. The bitcoin core developers see the need for a hard fork, announce the hard fork, and ask everyone to update. If the majority of nodes agree than the hard fork was successful. So in Stellar's case, they could be asking people who run validating nodes to update their software, or to simply modify their UNL to point to only a single node (similar to seeding your bitcoin client with only a single peer). I think (and hope) that the Stellar Foundation doesn't have the ability to actually force people to de-decentralize.