Bear in mind it doesn't say that you can't use your healer gear to go Shadow, just that Spirit is going to be a wasted stat, like using a lot of +Hit stuff in your healing... which you shouldn't be.
I'm not really able to effectively comment on how it will affect Holy Paladin healing, since my Holy Paladin (my first healer toon!) is only level 71 right now, but the whole point of it is to make it so that that style of healing has to change. From what I understand, they want to do away with the big heal spam style of healing, and implement "smarter" healing, as it were - were you have to watch your mana levels, worry about mana regen, and use various different healing abilities.
As for how this will affect current gear, that's the least of my concerns right now. These patch updates usually come out what, like a month before the expansion launches? Even if my itemization is a little wonky at that point, I'll have a 30% buff to everything in ICC for farming gear, if I still need any, to offset this, and if worse comes to worse - as much as I wouldn't like to have to - I can always replace my current gear with dungeon drops, etc. as I level. As far as I see it, although again my only healer isn't at 80 yet (soon!), I wouldn't worry too much about the impact these changes will have on WotLK, about all it will affect is whether or not your current gear will last you into the new Cataclysm raids or not.