I guess it does depend on the code(s). But those codes are there for a reason. Old buildings in CA don't meet modern codes, and those modern codes exist because a lot of those old buildings in CA fell down during earthquakes.

Sure. But we don't tear down every building in California built before the modern codes. The majority of structures there are going to be decades old, and won't comply with new requirements. But those buildings are considered safe for occupancy.

Even if the new ballroom - if ever it gets built - doesn't comply with current codes, that's unlikely to mean that it's not safe to occupy.

As I understand it, the older building predating the new codes is grandfathered. But if you try to renovate, you have to bring EVERYTHING up to code (i.e. the grandfather is nullified).

It varies by code and jurisdiction. A lot of codes use the "50% Rule" - improvements of more than 50% of the value of the building require bringing the whole building up to code. Renovations below that threshold don't trigger that requirement - the new stuff has to meet current code, but the existing stuff outside the scope of work can remain. There are tweaks (Florida uses a combination of valuation, area of work, and "path of travel" to determine where you have to update existing structures), but it's more common than not that a renovation does not trigger an obligation to bring the whole existing building up to current code.