Client conditioning is an important part of wedding photography. A good starting point is an open and honest relationship and an expectation that the client needs to choose what is right for them (not necessarily you) your photographic gear is of key importance and you need the ability to sell its benefits while also admitting any weakness. You need to let clients know there is no perfect camera and all have tradeoffs.
I would start off by explaining how all new cameras have significantly better IQ than the best from only a few years ago, yet they could still take great photos then(you could add that pentax and nikon use the same sony chips in many cases) The most important thing that a wedding photographer needs is familiarity, redundancy, and durability. I would then talk about your choice of gear, whether it be you years of use, your preference of colour rendering, features such as weather sealing or lens choices, and last but not least the trade off between sensor size, weight and performance. You should tell them that a wedding day is a marathon for a photographer, and a big huge heavy camera may take marginally better photos but a exhausted photographer will not. You could mention how many pros are moving over to fuji for this reason. I would also mention the strength of FF, if you need to shoot in a dimly lit chapel without flash photography then I would not hesitate to recommend that they should find someone with a FF and fast glass.
The most important thing is to convince the clients that you are acting in their best interest (by being honest). That will gain you more clients than changing camera brands. It will also increase your proportion of sensible clients!