Still, I wonder how to regenerate the change address if this happened again... It might be useful to other users todo 
An address expresses a public key with some added meta data. There can be several addresses for a given public key. In the case of Armory, there are three. When you get Armory online, or when you take a transaction to an offline signer, Armory will detect which public key was used. That is because it carries all address variations per public key.
This change came later in Armory's life. At first Armory only had one address type, before P2SH was a thing even. Therefor, Armory's GUI was not built with that kind of future in mind. When I added the new address types (that was the segwit support release), I opted to let the GUI display a single address per public key and detect which address type was being used in the background.
As a user, when you generate an address, it is assigned the default type as displayed in your settings. In Expert User mode, you can modify this address type on the fly in the new address creation dialog. In your specific case, you'd have to generate new addresses while you cycle through the 3 different types until you find what you're looking for.
