I'm trying to come up with a list of general grandfathering rules for 401(a) plans. As an example, post-88 earnings on elective deferrals are not available for hardship withdrawal. So, any plan that has been around since 88 (or earlier) would have to watch out for this. Any other specific examples or a source for a more comprehensive list?

One of my favorites: distribution rules ("investment in the contract") for after-tax contributions and the special implications for loans.