In the cases shown, yes there is a difference but it is small. In the first sentence, nouveau means new (recent). Some France residents could correct me with this but they sometimes replace it directly with neuf. So you were right to assume “if there’s a new development” as a meaning. As for the second sentence, nouvelles seems to be refering to new information like “I’ll contact you when I know more”. In a wider context like a missing person, one might say this to a family member while waiting for the police report.

Be aware that both meanings are really close. In fact, you could mix them up while speaking and people would not necessarily notice it (I wouldn’t).

So in short,

nouveau = something changed

nouvelles = new pieces of information arrived