Ah ok, they need to share the same fast_replaceable_group
value. Which at the moment with the 1.6.1 update, I am just modifying the vanilla display panel to share the same value as the plates, which is safe since the vanilla panel doesn't have this value set. The Larger Display Panels mod doesn't give their panels this value either.
I could if you think it would be useful, make it so the Larger Display Panels' 2x2 shares the same value if the mod is installed. So you can then quickly replace their 2x2 with a 2x2 Display Plate.
I'm not 100% sure of the etiquette around a mod modifying another mod though, which is funny to say since I do have another mod that only has the goal of modifying another mod.
That being said, 1.6.1 of this mod does include what you want. You can quickly place over an existing display panel with a 1x1 or a 3x3 and it will keep the same settings. As for the upgrade planner, no, that is a different system from what I can tell.