Suitability of 15400 as a management platform aside, you basically need to import the gateways as new devices. It might be a good time for a clean re-install of the software on them, just to start fresh - for the regular cluster you can do it one member at a time, just clean install one, create a new gateway cluster with just that fresh gateway, then do the other one. Unfortunately for VSX you cannot create a single-node cluster, and you won't be able to just re-SIC it as is, you'll need to rebuild it and recreate it all from scratch. If you can possibly borrow a suitable extra gateway to temporarily use to build the VSX cluster on the new SMS you can leave one of the existing cluster members running while you do the build, then swap it out afterwards.