Well, if it is up to code valid at build time, it is "up to code" now as well.
Yes, that is true, until you start remodeling. Then it counts as a new installation and must be upgraded. Many early installations used undersized wire, so this is probably where it came from.
Saying that Aluminum is more conductive by weight is not really helpful, because it is so light. By volume, which is how wire is chosen, aluminum is a worse conductor. A larger size must be used - generally two AWG less (smaller number is bigger) than the proper copper wire.
