Is there a contingency plan in the event of a catastrophic attack on AES? NIST selected Rijndael in 2000 to be AES. In a paper from the Serpent authors, they mention that there was the possibility of choosing a second cipher as a backup in the case of any severe breaks:


  I believe that there should be only one standard. NIST should decide on one standard in order to ensure that the standard is accepted and adopted as soon as possible. However, NIST can publish the choice of a backup cipher which will replace the standard in case it is broken or in case other circumstances (such as intellectual property problems) will prevent it from being used by the public.


NIST has not done this, according to section 2.4 of a report from NIST. Is there any process in place in the event that a severe cryptanalytic attack against AES is discovered? Imagine it's 2029 and a new paper comes out showing that all 14 rounds can be broken with a complexity of $2^{50}$ and a negligible amount of known plaintext. Would there be any official steps taken by NIST, for example changing the standard to use more rounds? Would one of the other AES candidates be chosen as AES2?

I don't believe that there will be a major break in the cipher without prior cryptanalysis showing it to be gradually weaker and weaker, so I highly doubt the total break, if one is ever discovered, will come as a surprise. However, I am still curious to know if there is an official policy on the matter.
I'm not aware of any official NIST policy on the matter, so I can only make educated guesses. 

I guess new algorithms have sprung up and are already in place. ChaCha20 is used in TLS 1.2 and 1.3. For hash functions, neither SHA-2 nor SHA-3 are depending on AES in any way. The sponge function in Keccak (SHA-3) can also be used as a symmetric cipher (Ketje, Keyak and Kravatte) and - with a bit of tweaking - as MAC (KMAC). So rather than to move backwards I think NIST would simply standardize on existing ciphers together with more modern ciphers based on Keccak.

Although I see merit in the answer of AleksanderRas, I personally don't think that one of the original AES candidates would be chosen as new AES (or FES, for fixed encryption standard). The world has moved on; there are likely more secure and certainly faster block ciphers around. For instance, I could see that Bruce Schneier and the Skein team would choose Threefish over Twofish. Possibly Serpent would make a chance if Rijndael is broken, as runner up with a high security margin. It does seem to have much in common with AES though, so maybe a break could also influence the security of Serpent, even if it does have a high number of rounds.

This brings us to an important final point: possibly a broken AES could simply be fixed by upping the number of rounds. In that case: the king is dead, long live the king. The hardware is there after all, and quite often the number of rounds can be configured. This would make the most sense in the short term and would allow NIST some breathing space to think of a replacement.