Well, as I had only been signed up for a month, I figured that ShareCG had been announcing they were going to make this move for some time, and I just didn't know it. Was this not the case, and the announcement truly was sudden?
In any event, even if it was, and was a breach in contract, I have to wonder if it's even worth suing over? Especially as they've said they were going to bring revenue sharing back when they've made it less prone to abuse (nothing is ever free from abuse, not when money is involved). Me, I'm content to leave the whole matter alone. I'm still going to upload my content here because, in the end, I've decided that I like ShareCG's format, especially the ability to upload nice, large pictures demonstrating what's in the zip I've uploaded.