First we'd have to know how Google determines whether or not a page has been updated. Do they compare their cache to the current page, or do they merely look at the date/time of the page on the server? I don't know the answer of that question. If they use page date, then all you'd have to do is re-upload the same page every now and then to trick them. Wouldn't that be nice?
Even though I think that free sites (the kind you submit) can be improved upon, I'm not sure that it would be worth the effort to apply updates manually. Preacher and I were discussing this and he mentioned the use of an RSS feed. One could employ one of those RSS2HTML gizmos and simply update content on their own niche-specific feed without having to touch the free site, which would avoid the fear of a handful of link list bots yanking your sites for changes. I wouldn't suggest using sponsor hosted feeds though.
As for big free sites/hubs - you could do the same thing, or put a block of mixed text and image content on a timed rotation. One of the reasons that SEs like real blogs is because of large chunks of text interspersed with a few images and semi-routine updates. Treat any hub the way you would a blog and you're probably on the right track.
As to how much of a change constitutes an update to Google, I wouldn't know. That's more up Halfdeck's alley. My limited information is based on how Google reacts to my pages that I'm constantly tweaking as compared the ones that I haven't touched in quite some time. When I make routine changes, the bot comes buy more often and hits more pages. I get more Google traffic to the pages that get updated, but that may have more to do with the content of those pages and not the fact that they are in a constant state of change. My opinion is that it's due to both the type of content AND that it is ever-evolving.