Bob:
The ASGCA has some kind of standard for when a "renovation" counts as a "new course" for the purposes of qualifying for membership in the organization, along the lines you describe. (Personally, I don't agree with that because it might convince a young architect to move around greens and tees needlessly so he can qualify for membership, but that's another story.)
However, I don't think the ASGCA polices whether one of their members can publicly claim design credit for those courses even if they don't qualify as "significant changes" under the membership guidelines. Rees Jones, for one, has taken credit on many courses where he didn't make substantial changes.
Cornish and Whitten's book included a lot of faux credits. Cornish listed every course he had consulted on, including Crystal Downs, where all he did was suggested capes in the bunkers (later shown not to have existed) and change the mowing lines to reduce fairway acreage. I don't really blame them, that was the first attempt at assembling all that info, and all they had to go on was the architects' own records or their word. Naturally they were all assumed to be gentlemen about such things and not to take credit for things they shouldn't.
Then there's the issue of whether a design associate gets credit or not, which is a huge can of worms. If you want to give them credit in today's world, there is no way to be consistent going back to the old days. And, frequently, the associate who most looks to gain some credit for his role isn't the one who really contributed the most on that project. As someone mentioned the other day, we would really require Hollywood "credit meetings" to sort out stuff like that. And nobody really wants to read a laundry list of names for the designer ... including the client, who is more interested in promoting the guy whose name he paid for.
If you figure out sensible rules for credits, and correct everything historically, I will be glad to abide by your new rules. But I'm pretty sure others will not.