Yes you can use them interchangeably, however happenstance sounds much more formal, probably because it is falling out of common use.

Happenstance was a common word, that would have been known in recent decades by anyone who spoke or read English. True, it is nowadays not a commonly used word. Some authours use words that are somewhat less common to make their writing more interesting. You would still expect to see it in their work. English has many words that are recognised, but less commonly used.

Wow a new word for me, "happenstance"what does it mean?