When an author leaves a comic, all of that author's Author Comments are deleted. There's no warning about this, and there's no way to have it not happen, other than by going through every page and manually setting the comments to be regular comments.
I think upon leaving a comic, an author should be given the option to choose one of three things to do with any Author comments they may have on the pages:
Delete all author comments. This is what currently happens.
Downgrade all author comments to regular comments. This would break any advanced HTML in them (links, DIVs, lists, etc), but it'll preserve them, at least.
Leave author comments as author comments. I don't know if that's possible, but if it is, I'd love to see this option. Many of my comments on comics I participate(d) in are quite relevant and should be at the top, but staying with a comic just for this is a pain. Given that one has to be an author to add them and I don't see any reason to check whether that person is still an author, I think it should be doable.
I think that if another author kicks an author out, they should be given these same options for that person's comments too.